The moment that changed Declan Murphy’s life for ever arrived when he was 28. It was in 1994 at a May bank holiday meeting at Haydock Park, and Murphy, a celebrated jump jockey, was at the pinnacle of his career, riding Arcot, the favourite in the Swinton Hurdle. Heading into the last hurdle, the pair misjudged their stride and fell. While Murphy lay unconscious on the ground another horse galloped over him, one hoof hitting his head and shattering his skull in 12 places. It was an injury so severe that within days the Racing Post would run his obituary under the stark headline: “Declan Murphy dies in horror fall.”

Twenty-plus years would pass before Murphy, who spent four days in a medically induced coma and came within hours of having the life support machine switched off, felt up to revisiting his ghosts. The result, Centaur, written with Ami Rao, is not only a certain candidate for the William Hill sports book of the year but also sure to be on many non-sports fans’ end-of-year lists. Emotional and honest, Centaur is an unflinching look at how Murphy “came back from the dead” and the heavy price extracted for doing so.

“I suppose the key is that I never really felt there was going to be a book,” Murphy, 51, admits. “It was more like therapy in that I was talking to Ami and no one else was there, so I could say anything and feel it was going nowhere.”

There was also a bombshell waiting to drop – a secret Murphy had kept from his family, his closest friends, his wife and his daughter: he could remember nothing of his time as a professional jockey. Four and a half years wiped clean on the day he woke up in hospital. “That was the hardest thing,” he admits. “The last thing I remembered I was in California as a 22-year-old studying at UCLA and riding horses in my spare time. I woke up and in my head I went from having this great life as an eternal student to not being able to move or do anything. I didn’t know what had happened or how I got there. I had no memory of those last few years at all.”

Did no one raise it with him? His answer could perhaps only come from someone raised in the sort of close-knit Irish family where secrets are routinely swept under carpets and difficult subjects carried silently to the grave. “My brother Pat tried to talk about my career a couple of times at the beginning but nobody ever spoke to me about my accident because I never gave them cause to, and because we weren’t talking about that, we weren’t talking about what happened before.”

There was more than just privacy at stake. Murphy describes his situation post-injury as “a three-way war between my body, my mind and my spirit”. It was an intensely private battle and one built on a precarious and ever-shifting base: to acknowledge the missing years would be to acknowledge that he might not recover.

He laughs. “And then I agreed to write the book with Ami, and after all that time I had to say: ‘Oh, by the way, I don’t remember the most important years of this book and I’ve never told anyone that I don’t remember.’ Her reaction was pretty much as though I’d said ‘there’s a tsunami coming, get out of the way, quick!’”

Declan Murphy in 1995, riding for the first time after his near-fatal fall. Photograph: Phil Cole/Getty Images

They pieced together the missing years by watching videos of Murphy’s rides, talking to people and reading cuttings, but there are some things that no amount of photographs or letters can return. Centaur is a book haunted by ghosts: lost careers, lost memories, lost friendships and, most devastatingly of all, lost love.

In the book’s most brutal chapter, Murphy and his then girlfriend Joanna both discuss their relationship, separately. “I look at the photograph of us together now and it makes me cry because we look so happy,” he admits. “In my head I think I would have done things differently if I’d known how she was feeling but the truth is I couldn’t have made that decision because I was in the throes of a mega-hell crisis and I never stopped to think about anybody but myself.” He pauses, choosing his next words carefully. “Because I had to make that choice between myself and the world, and I chose myself.”

He did so with a singular determination, convincing himself that there was no part of his injury, physical or mental, he couldn’t overcome, then closing himself off from his closest friends and family and refusing to let them in, even when things were at their worst and he would find himself screaming with nightmares or lying on the ground unable to move.

Does he think that the cost of his survival demanded a terrible price from those closest to him? “Absolutely. It was a sad and selfish thing to do but it was what I had to do to get to where I needed to get to. That’s how much I was prepared to lose just to gain my life.”

In the end what saved him was the conviction that all this suffering would pass. “So much of what I went through was the same as crawling through a dark tunnel riddled with danger with nothing but a belief that there would be light in the other end.”

Eventually there was: Murphy recovered enough to ride again. He did so just once, winning a race that even he admits it was “madness” to ride in. Then he walked away from racing. “Winning that race brought me relief, it meant I’d got through the tiny hole in the tunnel that I had to get through,” he explains. “But then I had to get as far away from being me as I could. I never wanted my life to be defined by my injury or by racing because in my head my life hadn’t begun yet.”

He moved to New York and built a comfortable new life as an investor and property developer. In 2005, at the age of 39, there was a marriage to Zulema, a Spanish woman he met at a party in London. The pair now live in Barcelona with their seven-year-old, Sienna. “It’s a successful life and a quiet life and I’m very happy,” he says.

Writing the book has drawn a final line between the love-filled life he has and the ghost of the life that might have been. It was a tough story to tell, and traumatic at times, but if there’s one thing he hopes people take from it is this: “When you believe in something you have to give all of yourself to get it. We are all capable of infinitely more than we think.”

Centaur, by Declan Murphy and Ami Rao, is published by Doubleday on 27 April at £16.99 To order a copy for £14.44, go to bookshop.theguardian.com or call 0330 333 6846