Books: Keeper of the flame : Later Auden by Edward Mendelson Faber & Faber, pounds 25, 570pp

What's the last thing a great poet ought to do? Choose the right literary executor. Sean French rejoices that W H Auden made sure his work rests in good hands the right choice we knew about them. T H Barrett finds holes in the Wall holes in the Wall holes in the Wall

In the couple of years before W H Auden's death in 1973, the heavy smoking, drinking and pill-popping caught up with him. His face looked like a geological relic. David Hockney made a memorable drawing, commenting: "I kept thinking, if his face was that wrinkled, what did his balls look like?" His conversation was repetitive; he became isolated and depressed.

But even while so much else was going wrong, Auden did a brilliant thing. He had got to know Edward Mendelson, a young Yale academic, and enlisted his cooperation in editing a collection of essays. He was so impressed that he appointed him as his literary executor.

The result of this choice has been startling. Compare the posthumous publishing history of Auden with that of T S Eliot, under the supervision of his widow, Valerie. In the 35 years since Eliot's death there has been no proper revision of the collected poetry, no expanded selection of essays. The edition of the letters seems to have stuck at volume one, and there have been a number of unseemly squabbles with unauthorised biographers.

On the other hand, Mendelson has devoted much of his career to producing a series of superbly edited editions of the poetry, prose, plays and librettos. These range from daunting (and scarily expensive) scholarly works to the indispensible selection of the poetry available as a cheap paperback. He has also cooperated tactfully with biographers, critics and other editors. The result is that an extraordinary amount of Auden's work is accessible (with even more, such as wonderful late essays and reviews, to come). It's difficult to think of any editor who has done as much for his subject since Max Brod refused Kafka's deathbed request to destroy his manuscripts.

Mendelson has also acted almost as Auden's authorised critic in various introductions and notes and, above all, in Early Auden (published back in 1981) and now Later Auden. This is not the slavish, sycophantic process it sounds. From the very start, when he was an undergraduate poet, Auden led a controversial public career. He constantly did things that great poets weren't supposed to. He collaborated, wrote in unrespectable forms such as commentaries for films or nightclub songs, made private jokes in his poetry, wrote about gasworks and disused railway sidings, and claimed to be bored by nature. He kept developing and leaving supporters adrift.

And this was before, with Christopher Isherwood, he left Britain for America just before the outbreak of the Second World War. The ferocious attacks on this supposed act of desertion mutated into an attack on his poetry. Philip Larkin, for instance, wrote a devastating review describing the American Auden as "too verbose to be memorable and too intellectual to be moving".

Auden emerges from this book as a man entirely lacking any kind of spite. Mendelson tells us that just a few months after Larkin's review appeared, Auden reviewed Larkin's The Less Deceived and "praised it without reservation".

Auden's growing tendency to rewrite, cut or suppress famous early poetry caused even more resentment, even among close friends. Mendelson uses his knowledge of the published and unpublished work to trace the logic of Auden's development. To put it crudely, this amounts to moving from being a poet who wrote what was beautiful even if he didn't really believe it was true to one who tried to get at truth even if the result might not be conventionally beautiful.

The knowledge needed to trace Auden's shifts requires awesome scholarship. Nobody has ever mistaken The Age of Anxiety for a simple poem. Larkin said that he had never got through it, or met anyone who had. In a typical detail, Mendelson mentions that many critics have pointed out that the poem's epigraph comes from the Dies Irae sung in the Mass for All Souls night. But, he adds, "the relation of these details to the rest of the poem makes sense only in terms of the little-known book in which Auden found them". Mendelson goes on to demonstrate with great subtlety the way in which Auden drew on Out of Revolution, an "eccentric, panoramic study of history" by the forgotten Eugen Rosenstock-Huessy.

I don't think Larkin would have been won over. Mendelson doesn't give much attention to tone, the way that the American Auden just sounds so different from the English Auden. But a serious reading of masterpieces like The Sea and the Mirror, For the Time Being and Horae Canonicae now starts with this book.

There are lesser compensations to Mendelson's thorough scholarship. On page 397 he decorously avoids mentioning the name of the young man addressed in "Lay your sleeping head, my love". But if you cross-reference it with the note on page 796 of Mendelson's edition of Auden's prose, you can, for the first time, work out who was the subject of one of this century's most beautiful love poems.