Philip Gould is boiling the kettle, chatting casually about football, when he says something that chokes me. "You know, this period of death is astonishing." The once-imposing spin doctor looks terrible – cheeks hollowed, jeans unfilled, hair lank, a tube inserted into his stomach to feed him – but is talking with such tenderness, such love and hope. "The moment you enter the death phase it is a different place. It's more intense, more extraordinary, much more powerful." He says it with almost evangelical fervour. Four weeks ago, he was told his cancer of the oesophagus had come back for the third time, and now there was no hope. He asked for a prognosis. Three months, the specialist told him. His wife, the publisher Gail Rebuck, then asked for the best-case scenario. Three months, the specialist repeated. Gould says in the days that followed, he and Rebuck talked and talked, about the past and future, about all they had got right and all they had got wrong; a period of reckoning.

In a way, he says, it's a privilege to be in his position – to have a deadline, to be given a chance to sort everything. "I do really feel I know where I am now." Don't get me wrong, he says – he has loved his life, wishes he could have enjoyed pottering about in old age, hates the chemotherapy, but it's not all negative. He's writing a book about his cancer – the initial diagnosis, its recurrence, and now he's in the final stage. "Death is not discussed very much, but I will write about this. I'll finish the book."

Is he scared of dying? He shakes his head. "From the moment I resolved and reconciled things with Gail the fear went. I don't feel I've got any fear now. I think acceptance is the key. If you accept death, fear disappears." The book, to be called The Unfinished Life, will be published by Rebuck's company Random House – almost certainly posthumously.

Gould, 61, has spent a lifetime spinning and strategising politics. He is one of the masterminds behind the creation of New Labour. Indeed, in his newly updated book The Unfinished Revolution he claims he was the first to coin the phrase way back in 1989 though today he's less certain – it could have been Bill Clinton, could have been Alastair Campbell, he says. Whatever, Gould was there at the start. At the beginning of the book he talks about being born into an aspiring middle-class family of teachers (his father became a headteacher) in Woking, Surrey, and how he and his peer group felt stymied by traditional Labour – yes it was their party, but they had ambition, they wanted to own property rather than rent council houses, they wanted to be helped in fulfilling their potential rather than nannied by the state.

He went to grammar school, left with one O-level, regretted his laziness and started again. He ended up with a good university degree, a brilliant businesswoman for a wife, and a job in advertising. He left advertising to form a consultancy with the pollster Deborah Mattinson. They introduced the Labour party to focus groups, realising it wasn't enough for politicians to lecture from on high; they had to listen to what the people wanted and then see how they could deliver it. By the mid-80s, Gould was working almost exclusively for Labour. His life project, his purpose, it seemed, was to make them electable once again. He succeeded, of course, and in the process was ennobled and came to be regarded by many of Labour's premier players as a sage – a confessor figure, even. He was there to witness the fall, rise and fall of Labour, forever sounding out opinion, and returning the findings to the party.

Today, he wants to talk about all he has learned, but it's a very different story to the one he expected to be telling. Not so much about high politics, as the intimacies of family life, friendship and love. All his life, he has been so careful about what he has said – weighing out every sentence by the teaspoon for nuance. Now he's unburdened. And the subject he returns to again and again is Rebuck. They have known each other since they were students at Sussex University in 1971, and married in 1985. He has always been besotted with her – and more than a little insecure. Insecurity is a running theme in Gould's life. Although there was the peerage, and the prominent position as leading Labour thinker, there was also the memory of early academic failure, the knowledge that he had never managed to make money and had always relied on Rebuck, the sense that she was too good for him, and worst of all the crippling doubt that she had never truly loved him. Ultimately, you sense, while he is obsessed with politics, and adores his two grownup daughters, Georgia and Grace, it all comes down to Rebuck.

"It was only when I got my diagnosis that I realised how much she loved me," he says quietly. Again he mentions the intensity of these last days. I've not heard anybody talk about death so movingly since Dennis Potter's interview with Melvyn Bragg. Unlike Potter, Gould is not on morphine today. If anything, he's high on life. And death.

Before I met Gould in person, he told me that if I had any prejudices about New Labour, the home he lives in would compound them. It is a magnificent regency house overlooking Regent's Park in London, with brightly coloured paintings covering every wall on every floor. He could easily open the place up as a gallery. The house is the ultimate in moneyed hippydippydom – candles at every corner, trinkets on every shelf, elephants from India, giraffes from Africa, memorabilia from their travels. After all, they are children of the 60s – at university, he says, the radical students would roll spliffs in lectures and run around naked in the rain.

He is staring out of the huge windows as we talk. It's a gorgeous late-summer day, and he's drinking in the light. "I live by the day. Just sitting in the park, looking at the flowers thinking how beautiful they are. It's almost … not hallucinogenic but it's a much stronger feeling than previously. For me, at the moment, going for a walk in the park with Gail is heaven."

Why had he been so uncertain about Rebuck's love for him? Well, he says, looking back, he did his best to mess up their relationship. "I was always putting politics first in a mad way, sometimes in a destructive way." But Rebuck is hardly workshy (she is the head of Random House, earns an estimated £1m a year and signed her contract in the hospital delivery room after giving birth to Grace). "Yes, but Gail was tremendous at coming home at 6pm every evening and reading to the girls. And she was always making houses wonderful, and I'd be dissatisfied. I am quite a destructive person. All those political people were …" He searches for the word. Mainlining politics? "Yes, and Gail hated that." In what ways did he put politics first? Numerous, he says – if they were having dinner and he had a brainwave, he'd wander off to develop it; he'd be out doing focus groups at weekends when the girls wanted to be with him; he insisted they moved to north London so the kids could go to a decent comprehensive when Rebuck would have preferred to stay where they were and go private. "I think of all the things she held against me, it was moving them to another place because I wanted them to go to a state school, and also the election was coming up and so on. Gail wasn't very happy." Rebuck told him politics had given him cancer, and he says it might well be true.

For so long, he says, he took things for granted. "When I thought maybe I've just got a few weeks, I thought God this is what they mean by the reckoning. I've got to sort all this stuff out in days. Is it possible to sort out all those things in your past that you'd prefer not to have done?" He's asking himself more than me. "But actually by going through the process we've gone through, I think we have. And it's a real blessing to have this time do that."

After Gould was first diagnosed, he found religion, then lost it after the cancer recurred. "When I was in that intensive-care unit, seeing the suffering, and feeling the pain myself, which was excruciating, I did lose a bit of faith." Since the cancer recurred for the third time, he has regained it.

The illness has changed him in so many ways, he says. Not least politically. The story of Philip Gould's cancer could be a parable. Here was the archetypal moderniser who had so lost faith in traditional Labour values that he took the private healthcare route. A surgeon in America told him he did not need the extreme surgery that the NHS had suggested. Gould took his advice and the cancer came back. By the time he returned to the NHS, it was too late. He's painfully aware of the ironies. "When I came back I began to realise that NHS facilities, particularly for this cancer, were fantastic. Now I wouldn't go to a private hospital. I have completely changed my view."

Has cancer changed his political position? "Oh yes. Certainly. No question." He's more old Labour? "Old Labour? It has certainly made me more aware … yes, it's made me more leftwing is the answer. It has made me realise the importance of public service and community. The other thing that has moved me is being in intensive care, which is really tough for the nurses. I don't know what they get, £35,000 a year? [The highest pay-grade is £34,189.] They do 12-hour shifts on one patient who is seriously ill and then they start talking about Wayne Rooney or whatever, and you realise with that level of inequality it's impossible to continue to get people to do these jobs because these jobs are based on the sense within society that there is some fairness about the level of contribution and the level of reward and that has broken down. So that changed me."

Gould has had a more complicated relationship with money than one might imagine. While Rebuck made a fortune, he struggled. Another irony is that it's only now he is dying that he is earning a decent whack as vice chair of Freud Communications. Until recently he was in debt, he says, and had serious money issues. I look at him – and the house – disbelievingly. How could he have been broke? Didn't he and Rebuck have a joint account? Now it's his turn to look at me disbelievingly. They had separate accounts? "Oh yes, of course. God, yes. Yes, yes of course. We had a small joint account, but basically we have our own accounts. My accounts were always very precarious. Keeping my [consultancy] business going was very hard." He pauses, and says he knows it sounds ridiculous to talk about money problems. "At the end of the day I own some of this house and Gail would have bailed me out, but I think she'd reached a stage where she'd had enough. And I really didn't want to dump her with money problems. Look I'm not saying in any normal person's lives I had problems, I am saying though that I didn't equip myself with glory when it came to making money, so Gail did keep me afloat, and I finally turned that around." He started at Freud in 2007, shortly before being diagnosed, and Matthew Freud has continued to pay him a salary throughout his illness. Gould says he is one of the few people he still sees a lot of.

What about his old political friends? Yes, he says, they are still there. If he had to have a heart to heart who would he turn to? "Oh Alastair," he says instantly. For years, his family and Campbell's family holidayed together, both men political junkies, and both reckless in their own ways. "We did leave a bit of havoc in our wake." At times, he says, politics bled him of his humanity.

He sees Peter Mandelson, David Miliband is regularly in touch, Ed Miliband called him the other day.

Isn't he disappointed by the legacy of New Labour – a project torn apart by personality and internecine conflict, not once but twice. No, he says, he doesn't see it so bleakly. "I think they [Blair and Brown] did tear themselves apart, but the question is how much did they achieve in the process? They did achieve a lot in those early years, and perhaps individually a lot in those subsequent years."

But surely he must have thought "not again" when Ed Miliband announced he was standing against his brother David? He nods. "Yes." Would Labour not have been better under David? He picks up a copy of his new book. "I'm almost inclined to read this bit out, and I will I think. 'I hated the conflict between the two, it upset me … It had been obvious to me for a long time that both brothers would stand, so as far back as the autumn of 2009 I approached Ed at a birthday party for Peter Mandelson and told him directly if there was a leadership campaign I'd vote for David first and Ed second. It was awkward, but I was determined not to make the same mistake as 1994; not say explicitly who I'd support … I did not try to stop or discourage him. He felt that whoever won it would be OK. I doubted that, but again I was quiet about it. I felt the ghosts of the past hanging over that conversation, and and I tried not to repeat the errors of history.'"

You're a good writer, I say.

"I do my best." He smiles and sips from his bottle of water. Does he miss not being able to eat? "God, yes." What food does he miss most? "Any kind of seafood. I love seafood. I love steaks of course. But on the other hand it is only a marginal loss because the other stuff replaces it. Your senses move elsewhere so you appreciate other things more."

If he was told he could have another 10 years but he'd lose the intensity of the present, would he take it? "I would not move. This is where I should be. I think, I think I should be here." Another pause. "But the moment I say that I think, what about Gail? Probably I'd take the 10 years because of Gail."

Of all his political friends, he says, Tony Blair has proved the greatest revelation. They had worked together for 13 years and had been close, but only in a professional way until his illness. "Obviously he's religious and we communicated on this spiritual level that changed our relationship completely, and made it very special. He contacts me on an almost daily basis, and texts me continually."

When the cancer returned for the second time, Blair told him that it hadn't finished with him, and now was the time for Gould to discover his purpose in life. And that is what he has been doing ever since: reckoning. And yes he has loved the politics, but he says it's time to let that go. In the end that has not been his chief purpose. So what has been? "The purpose now is just to live this life of imminent or emerging death in a way that gives most love to the people that matter to me, and I suppose prepares me for death."

As I leave, he says this might well be his final interview. Does that bother him? "No, it doesn't worry me at all. It feels fine." He smiles again. "On to the next thing."