I never had a book that changed me – not unless I count Wise Parenthood by Marie Stopes, when I was 11 years old. I found it on an inconspicuous shelf and supposed it would explain how my mother planned to raise us. In fact the book, written to teach married women about contraception, gave an exact account of sexual intercourse. Eve biting into the apple: that was me!

In my family it was taken for granted that if you were out of doors you were riding; if indoors you were reading. At every birthday I can remember, almost all my presents were books, and when I ran out of my own I turned to the grown-ups’ shelves. It might truly be said that books made me, but it was a continuous process, not (with that one exception) a matter of sudden illumination. The best I can do is think of which books I couldn’t let go if suddenly ordered to clear my shelves. There are four.

The first two are obvious: War and Peace and Middlemarch. All novels create places and people; those two create whole worlds. To both I return repeatedly. In Tolstoy’s case it was some time before I bothered with War, because the love stories were more important to the teenager I was then, and even in my twenties I rather sheepishly edged back into Peace about halfway through. Not until much later did I see how wonderful the War part of the book is. Now the only parts I skip are the few passages in which Tolstoy pontificates. The more one knows about his life the clearer it is that in many ways he was maddening, and there are tiny chinks in the book through which this can be glimpsed; but the miracle is that in War and Peace his genius prevailed over his human failings.

Middlemarch is also endlessly re-readable. So many stories are woven together that at each reading you seem to become involved with someone passed over only lightly during earlier readings. Unlike Tolstoy, George Eliot is someone who becomes increasingly likeable as one learns more about her, so her all-embracing but benign imagination is like a climate in which you remain after putting the book down, to ponder its suggestions and conclusions.

In choosing my two other must-keeps, I cheat, because although each can be described by one title, both consist of several volumes: Boswell’s Journals (four volumes) and Byron’s Letters (six). Both men did something unusual at the time when they were writing: they wrote as they spoke. Therefore we hear their voices. They are perfect examples of something marvellous: the power of the written word. Boswell died in 1795; Byron in 1824. Reading them, they are as much alive to us as they would be if they were talking in the next room.

Boswell is tricky to love, because he could be a bumptious ass and often skidded into disreputable behaviour, but he did have charm and intelligence – Doctor Johnson didn’t suffer fools gladly, and he came near to loving him as a son; and his sensible cousin Margaret, who knew him well, fell in love with him. He didn’t make her a good husband – he tells how when she was dying he bolted to London, and was appalled at himself for letting her die without him. And that is what is so amazing about him.

Over and over again he truly wanted to be a good man, making passionate resolutions to that end; and over and over again he did exactly what he was determined not to do; whereupon he sat down and described his own behaviour as precisely as a naturalist describes the behaviour of an animal he is studying – something a writer today might well attempt, but that no one did then. I believe his last years lacked the engaging optimism of his youth, but his journals, kept before he married, evoke every detail of his early days and show him to be unique in his appetite for life and his fascinated honesty in recording it.

Byron was a great letter-writer and his letters, some of them written when he was a precocious child, show how young he remained. He was only 36 when he died, and he had started out quite anchorless. He never knew his father, found his appalling mother a pain (as she was), and he had a humiliating deformity – a club foot – into the bargain. He had to make himself. Given great intelligence, wit and energy he quickly did so, creating a flashy, challenging persona. Crash, bang he went, through wild success as a romantic poet, the pathetic mistake of falling in love with his half-sister (the only person he felt to be his private own), and making a catastrophic marriage.

It would have taken him years to develop his potential, and he wasn’t given those years. He went a long way towards it before he died, but he never got closer than writing Don Juan and starting to find politics more interesting than sex. In his letters, however, he still lives, and whirls one through a series of fascinating experiences.

Having to live just one life, without the possibility of revising it, deleting its boring bits and polishing up its successes, has always seemed to me a meagre fate. My must-keep books have enabled me to experience lives other than my own: to enter other places, other times, other genders. And that is what the written word is for.