The Truth About Babies, from A-Z
by Ian Sansom
256pp, Granta, £10

Becoming a parent is like trying to build a boat while you are at sea. Constantly wet or calling for wipes, you navigate the first treacherous year of your baby's life, trying to establish some kind of structure, only to have it smashed by wind or its stormier sister, colic. The journey is exhausting. You have no idea where you are going, and the only sure thing is that there is no turning back.

Remarkably little of any power or depth has been written about this adventure, one of the greatest life has to offer, because in the past babies were the sole preserve of mothers. And it is in the nature of mothers not to have time to write stuff down. Worse, they are considered to have no experiences worth recording. There are scores of baby books out there, of course: antiseptic manuals offering tips on common rashes and the transition from bottle to solids. But what of the great mysteries of parenthood:

· Why have I started finding breakfast television interesting?

· Am I meant to love my child quite this much?

· Is it possible to exist on five hours' sleep without murdering someone?

· Will I ever read a book without pictures of cows in it again?

To these there has been no answer. Until now, that is, when men have noticed that they own babies too.

With its cover photograph of a dribbling tot, you could easily mistake The Truth About Babies for just another childcare bible. Once inside, though, you enter a maelstrom of tears, sick and epic nights - and that's just the dad. It is a world undreamt-of in the well-regulated nursery of Dr Spock. "I could not find a single baby book I wanted to read," Ian Sansom announces in his preface. "I wanted someone to tell me the truth."

You know you are in good, honest company the moment Sansom, a father of three, admits that his ambition was to produce a mighty, impressive work, but that he ended up with "notes and stories compiled some time after dark and before dawn", observations jotted down at bus stops or in the kitchen while his son was taking a nap. "When you think about babies you don't think small," he writes; "when you think about beginnings you soon get to thinking about immortality."

He is right: newborns bring all the big thoughts with them, but they demand so much attention that there's no time for philosophical treatises, only jottings and doodles in the margin. Life becomes fragments, occasional snatches of clarity in a milky fug of exhaustion, and it is this that Sansom has captured so brilliantly, moving between the numbing and the numinous, the trivial and the transcendent.

Part memoir of his son's first 12 months, part anthology of quotations on all things infantile, the book is arranged alphabetically from Advice and Anxiety, via Buggies and Buttons (poppers are best, he reckons) to Dribble, Fear, Fontanelles, Forgetting, Kisses, Nap ("the single most beautiful word in the English language"), Needs, Night, Rebirth, Regrets, Sentimentality, Shit, Vomit, Terror, Tidying, Weaning, Womb. If you think that sounds like a poem, you're not far wrong. Poems are the only kind of literature our new dad has time for any more, and the baby boy whose power he struggles to articulate sounds like a sonnet in human form - beautiful, compact and dense with hidden meaning. (Throughout the book, like Shakespeare writing to his beloved, Sansom addresses the infant simply as You.)

Conventional childcare books tell you what you must do to your baby; Sansom reveals what your baby will do to you. There is the catastrophic impact on appearance: "I have begun to look like a dad. I am wearing jumbo cords," he observes glumly. And on mental powers: "You demand attention. But attention is one of the many things I no longer have: fresh shirts, clean sheets, dignity. My attention span has gone." Perhaps, but his brain is still lively enough to register both pathos and bathos: "You look lovely. I pick you up to cuddle. From behind, you're soaked, from the neck down, in a mustardy slick of shit and piss."

Alongside the comedy of bringing up baby runs the hidden drama of resentment and depression, as the novice parent grapples with incarceration. In the section on Violence, Sansom confesses that one long broken night, when his boy was three months old, he began to shake him, but stopped himself in time. What is shocking is not the admission that he was almost driven to harm, but the fact that he could tell no one. Such feelings have no part in the stories we are allowed to recount to each other about family life. "The joys of parents are secret and so are their griefs and fears," said Francis Bacon. Four hundred years later, Sansom writes: "It's difficult to admit how beautiful you are... I can't even tell my friends."

Cyril Connolly famously described the pram in the hall as an enemy of promise. You wonder what Cyril would have made of wrestling a buggy off the Underground or stirring faeces-stained clothes in a plastic bucket in the bath. Ian Sansom belongs to the first generation of males to know their infants intimately, and the act of falling in love is as profoundly shocking to them as the chaos. Men like solutions - and babies won't be solved. As though to anchor himself in the whirlwind, Sansom turns to writers and philosophers for wisdom and consolation: Blake, HL Mencken, Henry Miller ("Anyone who hasn't had children doesn't know what life is.") The quotations are diverting enough, but does he really need to borrow resonance when he achieves so much through his own astounded observations? I prefer the baby trivia that the author has truffled up, probably during the long, stunned hours of darkness spent with a fretful infant in front of the BBC's Learning Zone. Did you know that baby, in cockney rhyming slang, is a basin of gravy? Well, it would be, wouldn't it?

Mothers reading The Truth About Babies will smile: we have known this stuff for as long as there have been children. But we need not begrudge fathers the chance to write it down. If men have decided that babies are interesting, then, sure enough, babies will become interesting.

"I can't decide," Sansom says to his tiny son. "Are you broadening my horizons or obscuring the view?" It is the eternal question of parenting: do they steal your life or do they eventually give it back to you with magically added value? One of the great pleasures of this volume is sensing the author, a bookish, private kind of guy, experiencing all the exasperation and the indignity, but again and again coming back, helplessly and heartfully, to the central fact of the baby's miraculousness. "I find myself becoming increasingly sincere. I find myself becoming a victim of my own genuine feelings." He feels like an American.

Sansom has written the true and beautiful book about babies that he couldn't find in a bookshop. Every new parent should have a copy for their journey through that first year. For anyone lying shipwrecked with their offspring on the sofa, here is the good news: you are not alone.

· Allison Pearson's novel, I Don't Know How She Does It: The Life of Kate Reddy, Working Mother is published by Chatto & Windus on July 4