"Rosebud," croaks Charles Foster Kane with his dying breath, the mysterious utterance symbolic of what drives Orson Welles's antihero to become a monster of his time. A reporter embarks on a doomed struggle to find out what he meant by it  and similar questions are being asked about Simon Cowell in Sweet Revenge. What drives the TV music mogul? What does he mean by it all?

Whether or not you care for Cowell's output is irrelevant. The unavoidable fact is that TV audiences here and in America have spent a decade gripped by top-rating productions in which he is the linchpin, which should make this a story of the age as much as of the man. Cowell has had an extraordinarily sustained ability to gauge and manipulate public taste. Others cast him as a kingmaker  a Sun splash on election morning claimed his endorsement for David Cameron, while Gordon Brown agonised at a perceived transfer in his affections. The assumption is that there must be more to him than meets the eye. What does he want with us?

Revenge, is the conclusion of Tom Bower's biography, for which he has spoken to a range of sources and been granted 200 hours of time with Cowell himself. Revenge against the music industry snobs who sneered at him when he was producing novelty records, but most of all, revenge against American Idol owner Simon Fuller, with whom he fought a legal battle over who got credit for being "creator" of Pop Idol (Cowell vowed to create a rival programme). The list of Fuller-crimes is as endless as it is oddly evanescent. Perhaps the only things more forgettable are most of the artists produced by Cowell's shows, whom the public embrace feverishly, then drop in the cold light of the off-season. A bit like in A Midsummer Night's Dream when Titania awakes and realises she's spent the night copping off with a donkey.

Simon Cowell does not cop off with donkeys, he'd very much like you to know. Contrary to the second or third most enduring rumour in showbiz, he is straight. Look! A Page 3 girl! A stripper! Sinitta! Yet puzzlement remains, and Cowell's fervent assertion that he is drawn to "crazy women" elevates the troupe of exes who have become his closest friends into something more intriguing than they are. They are neither super-gorgeous in the classic "trophy" mould, nor do they appear to be remotely amusing or interesting. They tend to be flatterers, and to varying degrees financially dependent on him. Bower observes the sex-free harem seconded to his holiday yacht. Dinners at these ladies' favourite restaurants are granted on the proviso that: "We'll speak about general matters for the first five minutes and the rest of the time we'll speak about me."

Meanwhile, Cowell's own body is a temple at which he is the most fanatical worshipper. Whether he is a sexual narcissist in the clinical sense one can't say, but his most recent major relationship was with his own make-up artist. Quelle surprise to learn that he talked to some Swiss "scientists" about cryogenically freezing his corpse after death for later reanimation. (The idea is shelved when he hears unfavourable reports about the clinic.)

Cowell's entire life seems to be an ersatz version of something else. Professionally, his X Factor is a near-copy of Pop Idol, while privately almost every one of his serious girlfriends has previously been out with his brother or a close friend. Cowell doesn't tend to pinch them; he merely takes up with them some time after the original relationship has run its course. Even his love affairs are simulacra. But then this is a man would always rather listen to those two actors from ITV's Soldier Soldier sing "Unchained Melody" than the Righteous Brothers, and who'd always rather hear a poignantly-backstoried teen fail to master "I Will Always Love You" than he would hear Whitney Houston's version.

His lack of imagination is by turns stunning and comic. The embodiment of the cliche that money's just a way of keeping score, he predictably begins holidaying on superyachts and commissioning ghastly sounding minimalist houses. He "hates vulgarity", apparently. The ex-girlfriends aside, he seems to have about two friends, at one point describing Louis Walsh  less a man than an X Factor-plot device, thanks to Cowell's multiple sackings and putdowns  as "my best male friend".

Of particular amusement are peripheral characters such as his relatively new friend Philip Green, the Topshop boss who, for all his billions, is clearly a starstrucker who realises that retail is showbiz for people too ugly even for politics. Green flies across the Atlantic to negotiate TV deals on behalf of Cowell for no fee, evidently just grateful for the chance to rub up against his world.

As for Cowell's own motivations, the revenge narrative that so dominates the book is not wholly convincing. At his last American Idol wrap party, an anonymous "curious bystander" overhears Cowell spelling it out to Fuller with sledgehammer simplicity: "All I've done  Britain's Got Talent, The X Factor and much more  is revenge for what you did to me. And there's much more to come."

Then again, perhaps people really do talk like this, because Cowell is nothing if not assiduous about making his personal semiotics accessible to even the most illiterate observer. Journalist visitors to his homes are frequently shown lots of mirrors and sparsely hung wardrobes, and dutifully chronicle both his "vanity" and "surprising austerity". Thus Cowell comes across as a man half-drunk on the power of working the levers of public taste, and half-depressed at how easy people make it for him. Only last week TV hosts were lapping up the spectacle of publicity ingenu Max Clifford wringing his hands about all the tales of hot hetero passion "exposed" by the book. Even Cowell's PR games are camp.

Of course, the irony after this book's serialisation is that Cowell has never looked gayer, for all his faux-distress as having been revealed to have briefly had it off with Dannii Minogue (such a pink pound favourite that Popjustice once reviewed a single of hers with the exhortation: "Buy shares in amyl nitrate now"). You would think the issue is not what he does in bed, but why he feels the need to present his sex life as he does  but then you'd have overestimated the public (a mistake Cowell never makes). The Sun wouldn't have splashed on his conquests for six consecutive days last week if they didn't shift papers.

For my money, though, it would be nice if fewer of those 200 hours had been spent establishing when Cowell got off with Dannii, and more on his reflections on being such an epoch-defining popular tastemaker. The legendary US TV producer Aaron Spelling is briefly cited as an inspiration  one doubts Cowell has heard of MGM's Irving Thalberg  but Cowell differs from his notional predecessors in his desire to be on camera himself.

A parentally indulged child whose nickname was "Mummy, look at me", he rose to orchestrating TV shows that make Triumph of the Will look like the first rehearsal of a playschool nativity play. Bower does unearth grimly entertaining tales of the shows' micromanagement, detailing the board-level discussions that take place over such characters as "a computer nerd" who enters American Idol. "I want him," says Cowell. "I want someone I can pick on." Such are the creatures against whom this powerful multi-millionaire chooses to define himself.

But why? Alas, Cowell's Rosebud eludes this book. Perhaps even now the flames are licking round a metaphorical sled, whose symbolism will never be disclosed. Or perhaps there is nothing there  a vast nullity at the heart of the man able to hold so many in his thrall. Perhaps the word he utters with his dying breath will be something like "sausages" or "Sinitta". Still, Welles said the Rosebud device was concocted as "the only way we could find to get off, as they used to say in vaudeville"  and Bower accomplishes that task with a wry final page detailing Cowell's enthusiasm for colonic irrigation. We have not yet got to the bottom of Cowell, but we have certainly got to Cowell's bottom.

 Marina Hyde's Celebrity: How Entertainers Took Over the World and Why We Need an Exit Strategy is published by Vintage.