Mr and Mrs Brocket, suburban Sydneysiders, are rigorously, abnormally normal. Unorthodoxy  vulgar attention-seeking  is anathema to them. So it is ghastly when their third child, Barnaby, from the moment of his birth, offends against the first and most basic of all conventions: the law of gravity. Unless weighted or tethered, Barnaby floats. He simply cannot keep his feet on the ground. This forces the Brockets into embarrassingly eccentric domestic arrangements, such as attaching mattresses to their ceilings. Taking the boy out for a walk, what with him floating at the end of a string like a kite, is shaming.

After eight years, the unbearable lightness of Barnaby Brocket drives his parents to do the Terrible Thing. They let him go. They empty his rucksack of sand and let him float up, up and away. Barnaby thus embarks on a picaresque circumnavigation of the globe in which he meets, among others, an elderly pair of lesbian balloonists, a Brazilian girl rejected by her father on account of her being pregnant and unmarried, a disinherited artist manqué who cleans the windows of the Chrysler building, and an ill-assorted bunch of astronauts. All have something in common, which they share with Barnaby: they have been disowned by their families because they are not "normal". Despite which they are, variously, loved, brave, generous, talented and happy.

The book's message appears most nakedly on page 92: "Just because your version of normal isn't the same as someone else's version doesn't mean that there's anything wrong with you," says Marjorie, one of the gay balloonists. An inarguable credo, entirely appropriate to young readers for whom adolescence looms. And one that Boyne repeatedly hammers home.

This is a rather frustrating book. It's unashamedly and often delightfully whimsical. It's lovely to look at (with jacket and illustrations by Oliver Jeffers), features a family dog called Captain WE Johns, is full of sly jokes (some too sly for children of 10 or thereabouts) and one gleeful thudder of a bad joke on page 247. Its language is frequently challenging, which is no bad thing. It has much of the pell-mell what-the-hell-happens-nextness of Dahl and Ibbotson.

There's a pleasing inversion of the unrelenting theme when we discover the reason for Mr and Mrs Brocket's lust for normalcy: as children, they had been forcibly groomed for stardom.

Yet, when reading a book, do you not sometimes wish you had been its editor? Had you been, in this case, you might have redacted solecisms such as the presence of foxes in Zambia or a marble sign "pinned" to a wall. More usefully, you might have cut at least 50 pages. There are a taxing number of them in this novel, which is episodic rather than plotted. Two or three fewer episodes would have given it more pace and punch without detracting from the narrative or its appeal for tolerance. It's a book just too heavy for its weightless hero, too ponderous for its uplifting message.

Nervousness on the part of an editor when confronted by the latest text from a bestselling author is understandable. I can imagine that whoever was given the job of editing Boyne, whose previous children's book was The Boy in the Striped Pyjamas, might have felt a touch pusillanimous. I wish she or he hadn't, though; a little lighter, this book would soar.

 Mal Peet's Life: An Exploded Diagram is published by Walker.