HE is approaching 80 and has read millions of words, probably written millions too.

It is the sort of backstory that gives the writer a sudden realisation that if one can not please everyone then at least one ought to please oneself. John Carey, Merton professor of English Literature at Oxford University, author, critic and reviewer, has done just that. This is not the book that was proposed to him, it is not the one his fans would expect, but it is the one he has produced somewhat erratically, oft times brilliantly and occasionally tediously.

Carey admits The Unexpected Professor was first envisaged as a history of English literature but he states in the foreword: "I've written something more personal - a history of English literature and me, how we met, how we got on, what came of it." This relationship has been passionate and all-consuming for Carey. Its retelling, though, has moments of banality amidst the customary cleverness of Carey as an insightful reader and witty yet profound writer.

However, there are passages that remind this reviewer of his early flirtation with the Jennings novels, the chronicles of a public schoolboy. There were constant references to the precise workings of a preparatory school that baffled the working-class Glaswegian. There were midnight feasts, housekeepers, matrons and allusions to the first remove. The first remove? That was what we called the rite of passage known more colloquially as eviction.

Thus when Carey speckles his prose with the minutiae of Oxford - the deans, the scouts, the wardens, the quads - one inwardly groans. He can protest with some justification that an account of his life in Oxford must include such terms but could one ever imagine a writer as fresh and intelligent as Carey writing the following sentence: "I organised a Faculty Seminar Day one Saturday in Hilary term, planned like a mini-conference.''

This is an almost precise summing up of the failures of the book. It is as if Carey has spent almost all of his adult life in Oxford, save a spell of National Service, and has become institutionalised. There are the drunken days as a student, the studying on Benzedrine and the roll call of lecturers who translate Greek poetry into Hebrew while playing chess with their cats. Or something like that.

These are, therefore, episodes when Casey invites the charge of caricature but he rises above this by dint of his authentic passion for literature. Just when one wearies over the precise use of a sconce-pot at an Oxford dinner, Carey reaches into the treasury of his formidable intellect and produces a gem.

One forgives everything because Carey is simply a reading obsessive and one with extraordinary, enlightening views. His account of life as a middle-class, grammar school boy is engaging and his National Service days are cleverly rendered. His upbringing in a quiet, enclosed home with a troubled brother is both moving and infuriatingly incomplete.

But it is when he talks of poets, rhythms and the sheer, wonderful, all-consuming joy of reading that this book offers evidence of Carey in excelsis. How is this for a declaration of the written word over anything else? Carey watches Olivier in a Shakespeare play, observing: "When actors start to waddle around and gesticulate it seems absurdly inferior to what your imagination has created from the words on the page.'' And later: "I prefer reading about things to actually seeing them".

This is the sort of confession one imagines should be made in the confines of a church hall over a cup of tea with other sufferers. Of course, there is a marvellous upside to Carey's addiction. He passes on learning, gilds every encounter with literature, produces essays and books that enlighten and entertain his fellow sufferers.

He is brilliant on Shaw: "He said big things very briefly and simply." His meetings with such as Larkin, Graves and Heaney reveal much of them as personalities but also point to their importance as artists. Carey, too, has the casual flourish that is, ironically, devastating. Kenneth Tynan, the celebrated critic, is described thus when he was nearing the end of his life: "He seemed like a bit of decoration left over from a party".

The judgments on writers and their works are made from a hinterland of reading and study that stretches back beyond the start of the Second World War but Carey is no lazy traditionalist. He dismisses Don Quixote with a brisk fervour that brings the sobering realisation a critic is never as right as when one agrees with him.

It is almost impossible not to fall into a reviewer's trap with Carey and review the book he has not written. The Unexpected Professor leaves much of family life unexplored, with his wife appearing in shadows, talking in whispers when she is patently and recognisably much-loved and central to his success. Similarly, much of the discussion on literature can seem lifted from lecture notes when Carey's short, sharp observations are more educational.

His final chapter speaks to this enhancing truth. So, In The End, Why Read? runs to just over two pages. It will take minutes to read but explains a lifetime of peering at words. Reading encourages doubt, says Carey. There can be reservations about the whole of The Unexpected Professor but not about Carey's capacity to inform and enthral.