Over the course of 60 years, Christopher Isherwood wrote more than a million words of diaries; after his death in 1986 his companion of 30 years, Don Bachardy, decided they should be published - uncut. As the first volume appears, Christopher Robbins talked to him in the Santa Monica house the couple used to share
On the night that Christopher Isherwood died, Don Bachardy - his companion of more than 30 years - sat down in their home in California, on the edge of Santa Monica Canyon, overlooking the Pacific, and began to read his diaries. Bachardy started with the last entry and worked his way back in time.

"I'd got through the day and had done some last drawings of Chris after he was dead, and I was getting ready for bed. Without any premeditation I went right to the desk where I knew he had kept the diaries all those years. Like a dog on scent."

During the period of his deepest grief, Bachardy was led gently back over his life with Isherwood by means of a literary time machine. That first night he read for about an hour, and every day afterwards before he went to bed, always in reverse order. "And there were sleepless nights when I would get up at three in the morning and read for a couple of hours. I read them very slowly. It took me months. I didn't want them to stop."

There were 12 volumes - six handwritten, six typewritten. Dotted throughout were paragraphs of advice and personal comment, some planted 25 years earlier. "Don, I know you are going to be reading these words after my death..."

"It was a very peculiar experience. And it was exactly the experience I needed because reading his words was as close a contact as anything apart from his physical presence. It accentuated my loss, of course. It made me know in a very acute way what I wasn't going to have any more. But it was also comforting - a kind of sublime anguish."

Now, ten years later, the rest of the world can begin to read the diaries, albeit in the conventional order. Isherwood had no interest in seeing his diaries in print during his lifetime, and the decision to publish was taken by Bachardy, who demanded that they be uncut. Volume One (1939- 1960), meticulously edited by Katherine Bucknell with index and glossary, is just published, and an exhibition of Bachardy's drawings of people who feature in the diaries - from Francis Bacon and Cecil Beaton to Igor Stravinsky - is on show at the National Portrait Gallery.

Keeping a diary seems to have been encouraged from a very early age by Isherwood's mother, Kathleen. At the age of six he dictated to her a tiny book called "The History of My Friends". Later, and for 60 years - until a month before his 79th birthday - he wrote diary entries several times a week, although most of the pre-1939 diaries have not survived. He records his own, self-searching obsession: "Who are you - who writes all this? Why do you write? Is it compulsion? Or an alibi - to disprove the charge of what crime?" (June 1958), and Katherine Bucknell's excellent Introduction describes the compulsion: "Isherwood wrote in his diary to provide evidence, week by week, that he was neither wasting his life nor spending it in the wrong way: that he was paying attention, that he was doing something of value ..."

The first volume opens with his emigration to the US with W H Auden, on 19 January 1939. They are seen off by E M Forster who asks whether he should join the Communist Party. "I forget what I answered. I think it was, `No'." The volume ends with the last handwritten entry on 26 August 1960, when Isherwood records celebrating his 56th birthday (Don gives him a Brooks Brothers shirt and a pair of barbecue bellows).

There is not a page that does not contain a good joke, original insight, deadly accurate description, or delicious nugget of gossip. He met every major literary figure of his time, and wrote scripts in Hollywood during the so-called golden age, so that he mixed with many of the stars. He lived in a large world and enjoyed a wide circle of friends - a very mixed bunch indeed. He went to parties where Bertrand Russell (a "monkey-gland lobster in a woolly, toy-sheep wig") rubbed shoulders with Ronald Colman, and "dumb cluck" Garbo hustled a somewhat seedy-looking Krishnamurti for spiritual enlightenment. "Kittenish in a rather embarrassing way ... she wanted to be told the secret of eternal youth, the meaning of life - but quickly, in one lesson, before her butterfly attention wandered away again." And then there's Charlie Chaplin on the set of The Great Dictator, confiding wickedly, "I'm prepared to sacrifice my last relative so that democracy shall not die."

He included anecdotes that amused him: "A good - because psychologically true - Hollywood story. A maid boasts to her friend - also a maid - about the house she works at: wonderful people - they entertain every night - and always big stars. The friend is thrilled: `And what do they talk about?' The maid: `Us'."

Almost as soon as he arrived in California, Isherwood embarked on what was to become a lifetime's spiritual quest into the Hindu philosophy of Vedanta. There's rather a lot of this, but Isherwood is such a natural sceptic that even the most worldly can maintain an interest. He originally dismissed those involved in mystical religion as "cranks, simpletons or sex maniacs", and once regarded the philosophy he was to adopt as "the ultimate in mystery-mongering nonsense". He abandoned an early ambition to become a monk, overcome by lust and repelled by asceticism: "Even the mildest kind of austerity affects me, somehow, like a cold breath from the grave."

The physical descriptions of the huge empty continent suggest that fundamentally America has not changed so much. As Isherwood leaves New York by Greyhound Bus - "a streamlined Martian projectile" - for California, he notes: "The road has eaten the landscape. Travel has defeated itself. You can drive at 80 mph and never get anywhere. Any part of the road is like all other parts ... Endless, flat-topped garage architecture." And on arrival in California, as he walks beside the Pacific, he observes "prima donnas of suntan frying slowly in their own grease".

He is hard on himself throughout, deploring his sloth and lust again and again: "I am lazy and dreamy and lecherous." He agonises over his safety in California during the war, admitting that his pacifism commingles with cowardice: "a very little danger goes a long way psychologically". He describes himself as "a sham, a mirror image, a nobody". After the self-serving literary memoirs of recent years, it is all very refreshing.

A major literary work, the diaries round off and complete the writer both as man and artist. They are intimate and intensely personal, the antithesis of Isherwood's detached and distant style as a novelist, where he deliberately takes on the role of an observer and chooses to see life through the lens of a camera. Strangely, the overall impression is of a man with a terrific sense of fun.

Two more volumes are scheduled. The second volume (1946-1952) is actually a reconstruction Isherwood wrote in the 1970s of a period in his life when he made few diary entries. It was a self-destructive, promiscuous, idle and hard-drinking time and, unusually for Isherwood, he recreates it with explicit sexual material. Bachardy refers to this as "the spicy volume". A third volume will cover the years from 1960 to 1983. Altogether the diaries run to a million words. There are few alterations in the original, and scarcely any awkward sentences or constructions. Isherwood polished and occasionally rewrote material up to the mid-1940s but, except for the reconstruction, the latter years are untouched. Even at his most unguarded and off-the-cuff, he is a stylist.

For the last six months of Isherwood's life, as he lay dying of prostate cancer, he was Bachardy's only subject. The drawings ruthlessly depict the suffering and anguish of a dying man, but even at the end Isherwood's mordant humour never left him. One afternoon Bachardy had been working in black acrylic and wash and had spread the drawings on the bedroom floor to dry. Unaware that he was observed, he picked up each one to check for wet spots. From behind him he heard Isherwood's voice: "I like the ones of him dying."

! Christopher Isherwood's `Diaries: Volume One 1939-1960', edited and introduced by Katherine Bucknell, is published by Methuen at pounds 25

! `Christopher Isherwood and his Circle: Drawings by Don Bachardy' is at the National Portrait Gallery, London WC2 until 7 Feb; free