He was the best company in the whole world; he had read widely and because he was an industrious man and filled with curiosity, he hoped to read much more. He would stay up late drinking and talking, moving with judicious and delicious care from the large questions of the day to the small sweet business of invective, anecdotes and gossip. You had to take full advantage of his company; he was never sticking around the next day. He had a flight to catch to a destination where he would denounce someone in need of denouncing, or to a debate where he would, he was sure, win the argument. And then he would stay up late again, and the talk would be brilliant.

Only once I saw another side of him. I dropped by his apartment in Washington late one evening about five or six years ago to find him alone. He had just eaten, and had a periodical propped up on the kitchen table. He was quiet, mellow, reserved, almost dull. The mischief was all missing. I wondered if he was like this more times than anyone, except his immediate family, guessed. As I left, it was clear to me he was settling in for a night's work, or a night's sleep. He was not himself at all, and I liked seeing that other side of him.

This last sad and oddly inspiring book comes with an introduction by his editor at Vanity Fair, Graydon Carter and an afterword by his wife Carol Blue. Christopher Hitchens's own pieces are shaped like a fugue; the theme is death, his own death, and the voice in each piece changes slightly as death comes closer. He begins simply with the theme: "I have more than once in my time woken up feeling like death. But nothing prepared me for the early morning in June when I came to consciousness feeling as if I were actually shackled to my own corpse. The whole cave of my chest and thorax seemed to have been hollowed out and then refilled with slow-drying cement. I could faintly hear myself breathe but could not manage to inflate my lungs."

Soon, it emerges that he has cancer of the oesophagus, the disease from which his father had died at the age of 79. Hitchens is only 61. It is clear that he will give anything to live. "I had real plans for the next decade … Will I really not live to see my children married? To watch the World Trade Center rise again? To read – if indeed not to write – the obituaries of elderly villains like Henry Kissinger and Joseph Ratzinger?"

And so the struggle begins; he writes with a calm and searching honesty about the idea that "I don't have a body, I am a body." As someone who liked a struggle, indeed often went downtown in search of one, he discovers that "when you sit in a room with a set of other finalists, and kindly people bring a huge bag of poison and plug it into your arm … the image of the ardent soldier or revolutionary is the very last one that will occur to you." While he loses his hair, he is rather pleased that "the chest hair that was once the toast of two continents hasn't yet wilted, but so much of it was shaved off for various hospital incisions that it is a rather patchy affair."

He writes then about how his many friends and his enemies respond to his illness. When someone writes to say that, on his death, he should "freeze at least my brain so that its cortex could be appreciated by posterity," he responds: "Well, I mean to say, gosh, thanks awfully." He offers a hilarious account in dialogue form of a woman coming to get a copy of his memoirs signed – he is on a book tour in the middle of all the treatment – and telling him about a friend with cancer who died an agonising death. He also manages to open a section of this book with a good new joke: "When you fall ill, people send you CDs. Very often, in my experience, these are by Leonard Cohen."

But other times here, he uses sentences that are too stark and serious to be aphoristic. He is in command of a new tone as he considers the damage done to his actual voice. "What do I hope for? If not a cure, then a remission. And what do I want back? In the most beautiful apposition of two of the simplest words in our language: the freedom of speech."

There is a great deal of suffering and slow loss as he undergoes every treatment possible. As his wife notes: "He responded to every bit of clinical and statistical good news with a radical, childlike hope." When such hope seems futile, he realises how much he is losing. With pains in his arms, hands and fingers, he writes: "Almost like the threatened loss of my voice, which is currently being alleviated by some temporary injections into my vocal folds, I feel my personality and identity dissolving as I contemplate dead hands and the loss of the transmission belts that connect me to writing and thinking."

The last section of Mortality is made up of "fragmentary jottings", which the publisher notes "were left unfinished at the time of the author's death". One of these notes reads: "If I convert it's because it's better a believer dies than that an atheist does." He does not convert of course. He remains true to the ideas that animated his life, and to the idea of what words can do.

In that last-quoted jotting, I can hear him starting up, ready for trouble. I think back to a few years ago, when there is no hint of illness. I am driving him to a party in Wicklow; at the party he will be brilliant and provocative and sparkling; also, he has just, he is sure, won hands-down in a debate about God in a theatre in Dublin. There is no one happier. He is full of beans and eager to spill them. "Christopher," I ask him, "what is the worst religion?" "Oh, Islam," he replies, in a tone both earnest and edged with self-parody, upping his English accent.

"And what is the second worst?" I ask. "Yours," he says slyly and leaves silence. We are less than a hour from our destination, but he will do everything in his power now to set up the argument so he can show me its shape, as in this book he does everything to make sure that his voice remains civilised, searching and ready to vanquish all his enemies, most notably in this case the dullness of death and its silence.

• Colm Tóibín's The Testament of Mary is published by Viking.