The Genius in My Basement by Alexander Masters

Cambridge is heaving with men - they're almost always men - whose minds have been blasted by the dynamite within them. At certain intersections the traffic knows to anticipate, and slow down for, wild-haired geniuses standing in the middle of the road. They look like homeless people and fit the common idea of what a pederast looks like (although they are usually - but not, in my experience, invariably - sexless).

Alexander Masters lives in the same house as Norton and has decided to write his biography.

Masters has experience dealing with the flotsam of society: his award-winning Stuart: A Life Backwards told the story of how someone became homeless.

This is a less harrowing book. Its subtitle is "the biography of a happy man" and it tells how the most naturally gifted mathematician in the country - he is still spoken of in awe by cognoscenti - turned into someone most of us would (carefully) cross the street to avoid. The result is not entirely a success. At one point he winkles out what the cause of Norton's decline was, the catastrophe that made him lose focus on the area of mathematics he was dedicated to. (Group Theory, and a subset of that, The Monster. Imagine a Sudoko square whose number of rows and columns is 54 digits long, and which exists in 196,883 dimensions. Can you imagine that? Good. Go to Cambridge.) It was grief, says Phillips. And what caused this grief? In Norton's own words: "Aaaaaahh, hunnh, ugggh ... The 1985 Deregulation of the Buses Act."

In other terms, this book is a complete success.

That is, in producing something that enlarges not only our understanding of Group Theory (Masters, although possessing a first-class degree in physics and applied mathematics, concedes that he is a speck in the distance compared with Norton), but something higher up in our priorities: an understanding of the human mind, and an enlargement of our sympathies. The reader at first all but recoils from Norton's eccentricity; but by the time Masters says "Simon's only mental pathology is an excessive desire to obey local housing law", about a third of the way through the book, we begin to see what he means. By the end of the book, whose length we might have, at the beginning, questioned (342 pages with this freak show), we have come just about to love him.

We certainly relish his extreme precision - he's always pulling Masters up about inaccuracies, and in so doing sparks off interesting reflections on the flawed nature of biography - his bizarre behaviour, his lack of egotism, the very noises he makes. Let Norton have the last words: "Aaah, I mean to say ... uuugh ... it is and it is not about fun.

"Oh dear. Can I go now, please?"