Plenty of people write novels at the age of 28 – zeitgeisty novels, slight novels, novels locked in place by their precocious youth – but few write one with the scope and maturity of Eddie's Bastard by William Kowalski. Two years shy of his 30th birthday, not long before the turn of the last millennium, Kowalski, a creative writing degree dropout-turned-teacher, gave us a debut which, while ostensibly a coming-of-age tale, takes in a sprawling family history going back from the 1980s to the American Civil War and beyond: a novel that in its rambling good nature and uncanny debutante's assurance, is as surprising, familial and immediately engaging as Kate Atkinson's Behind The Scenes At The Museum, yet strangely – in the UK, at least – had almost none of the impact.

I was originally handed a copy of Eddie's Bastard by the publishers of my first book, having been told that, as a fan of the good-natured comic epics of John Irving, I would love it. I put off reading it for years – something unusual, in my case, when it comes to a book with an ostrich on its jacket. Perhaps my stalling could be put down to a stubborn, half-secretly held belief that nobody writes a Great American Novel before the age of 30 any more. When I finally read it, in two days flat, I hoped to find that Kowalski had written numerous other full-length works of fiction, each just as funny but without Eddie's Bastard's smattering of callow flaws. What I found was that he'd written three more: the last, The Good Neighbour, appearing in 2004, since which he has been teaching at Nova Scotia Community College in Canada, breaking off only for screenwriting work and three short books for adults with learning disabilities.

This is sad because, in some parallel universe, one senses Eddie's Bastard's central character, Billy Mann, was the launchpad for a glittering literary career, as Garp was for Irving with 1978's The World According To Garp (Irving's fourth book, not his first, it's worth remembering). In the summer of 1970, Billy is deposited on his grandfather's doorstep in the upstate New York town of Mannville, along with a note with a shopping list on one side, and the words "Eddie's Bastard" on the other. After narrowly avoiding treading on him, his grandfather Thomas can immediately see in his face that he is a Mann, and soon works out that Billy is the son conceived by Eddie, Thomas's high school athlete hero son, shortly before Eddie was killed in Vietnam.

What ensues is a touching, if unconventional, relationship between grandparent and grandchild. Thomas is constantly sozzled, but his heart is good. If a three-dimensional man drank the amount of whiskey he does, he'd have been a far more cantankerous, suspect guardian, but Kowalski's writing has a fairytale aspect, leaning towards the belief that, if someone is nice to the core, circumstances and misfortune will do little to alter that fact.

With a brief bit of help from a vivid, rotund Dutch foster family who tickle strangers to get to know them (in many other books, this detail would come across as creepy; here, it just seems sweet) Thomas somehow brings Billy up to be a fine upstanding citizen: a voracious reader from an early age, who can think quickly in a crisis and heroically rescue his grandfather after a fall on the ice and, despite being undersized, stand up to the school bullies and to the abusive father of his childhood sweetheart, Annie ("the only pretty one" in the Mann's rival neighbouring family, the Simpsons). Actually, at times Billy seems a little too admirable, and this is one of Eddie's Bastard's few youthful drawbacks: its notions of good and evil can be a little too neatly trimmed. At the time of the book's publication, a New York Times reviewer called it "a happy pill that's a little too hard to swallow", which is a fairish criticism, but you can forgive Kowalski for his sentimentality when, for example, he describes the villainous patriarchal Simpson's face as looking "like a streambed in summertime".

As an orphan, from a once-great family, Billy can't help being an interesting character, but arguably the most forgettable element about Eddie's Bastard is his adolescence: his uncontrollable hard-ons, regrettable early forays into experimental prose, clashes with bigger boys, and his (albeit unusually tragic) teenage crush on Annie. Where the book really excels is in its digressions into the Mann past: the comic story of the ostrich farm that lost Thomas the family fortune and made him a laughing stock in the town that carries his name; the wise, increasingly eloquent diaries of his distant ancestor, Willie; the family curse that stretches back to the civil war. Best of all, perhaps, is the story of Thomas's time stranded on an island with a Japanese pilot during the second world war: a kind of good-natured version of John Boorman's Hell in the Pacific, with camaraderie instead of capture.

Witnessing Kowalski bringing all these strands back together is a little like walking in on a kitten sitting in front of a toasty log fire in a living room, amid several balls of wool it's played with a little too overzealously. You're tempted to fix it with a disapproving stare, but then you look at its face and any thought of a reprimand melts away. Also, the place is still kind of cosy: a good spot for a sit-down. Sure, it's a bit odd that we didn't find out until late in the book that Eddie spent his youth dressed exclusively in his dead father's clothes, and the ending is a little too neat, but we're being set up for similarly great sequel, which will take Eddie to New Mexico on a journey to find the mother he has never met. Besides, what other literary kitten writes a book with this kind of historical awareness?

Nick Hornby once described amazon reviewers as "bastards", and it's almost unheard of on the site for a book with more than 20 reviewers – no matter how critically acclaimed – to maintain a star rating of more than four out of five. In the anonymity of Internetland, there's always someone who can't resist putting the boot in. The brief popularity, in its own country, of Kowalski's debut is a decade in the past now, but, from 50 amazon.com reviewers, it averages 4.5 stars. Take a scan through them, and it becomes clear that here we have a bastard so lovable that even its fellow bastards can't stay mad at it.