He does it something like this. "Rosebud", you will almost certainly recall, is the word which bookends Citizen Kane, and seems to be the key to its mystery and its sense of squandered life. Kane opens with a sequence in which Welles's dying millionaire-tyrant whispers the name of the rose as his riddling last testament: it closes with the revelation, to the audience's eyes only, that "Rosebud" was the sled with which the child Kane had tried to fight off the man who had come to take him away from his mother and into his frigid, gaudy future. To the casual observer, this makes pretty good dramatic sense. The secret of Kane's life is that, like other damaged children, he never managed to drown out the echoes of an early wound with the clamour of adult accomplishment.
But, as Thomson first pointed out two decades ago in his critical study America in the Dark - the germ from which Rosebud has blossomed - anyone who thinks that this middlebrow Freudian pathos is the film's total burden or upshot simply hasn't been paying attention. If it said no more than that rich men are sad, how could it have earned its superlative status? (A status Thomson unexpectedly endorses here, justifies in page after inspired page of close reading, and ultimately finds harrowing.) Kane reeks of artifice, and among its themes is the recognition that "Fiction is the great virus waiting to do away with fact". Consider one odd detail, too momentous to be a mere slip. No one in the film actually hears Kane speak the word "Rosebud" - though there are fudging hints that the butler did it - which means that the quest to discover the essence of Kane's life not only has no end, it really has no beginning, either. Small wonder that Borges, connoisseur of labyrinths, admired Welles's work. Thomson's way out of the paradox is as fanciful as it is lovely: "The only coherent way to explain the form of the film is as the dying Kane's reverie."
We have been warned. Just as "Rosebud" is a solution that solves nothing, Rosebud is a life story that has attended to the deeper lessons of Welles's audacious imaginary biopic, heeding its warning that a resonant answer can make it harder to know the question. More alert to atmospheres, insinuations and "magical logic" than to the humbler drudgeries of chronology and proof, it is biography as reverie. You can gather some measure of its strangeness by dipping into the curious dialogues with an imaginary publisher - addressed with sprightly formality as "Sir", as though Thomson were Samuel Johnson or Sydney Greenstreet - sprinkled among its expository chapters. And you can infer its sense of mischief from the dialogue in which Thomson reimagines the episode of young, pre-famous Orson drinking and wenching his way around Ireland in 1931, and leaving behind him a small souvenir which develops into ... Peter O'Toole.
At which point the imaginary publisher starts to babble of lawyers, and literal-minded readers will start to champ at the bit, insisting that such flights are preposterous and that, like the cop from Dragnet, all we want are the facts, ma'am. But they will be misguided. We have facts in their plenty about Welles, in biographical books by Peter Bogdanovich and Frank Brady and Simon Callow and Joseph McBride and so on, most of which are worth reading, some outstanding. What we didn't have until now was a book in which a major critic gathers up all the records, sifts through the acres of Wellesian blarney which gave meat for a thousand talk shows (keeping him in large suppers through his Falstaff years), and then sees clearly, even frighteningly through them into the man's work, showing it to be far more desolate than we had dreamed.
Not that exegeses of Kane absorb all the book's energies. (Though the biographical form is plainly something of a commercial trojan horse for Thomson and other contemporary critics, who know that the punters don't shell out for Leavisite or Empsonian felicities but will swallow them quite happily if given enough passages on "those variously convex and concave curves in the white of a woman's body between the knees and the throat" - contours with which Welles was widely and intimately acquainted.) The other films and biographical matter are not just present and correct, but given giddying new spins, thus disclosing a "terrible solipsism" in movies as various as The Lady from Shanghai and Touch of Evil, and indeed in almost every act of Welles's life.
Thomson writes movingly on the legends surrounding the mutilated Magnificent Ambersons, the abandoned It's All True, the uncompleted Don Quixote, the sequestered Other Side of the Wind and the other runts and strays of Welles's later career, recognising that one reasons why they have been so cherished and dreamed over is precisely because they have the grandeur and melancholy of ruins. "There are creations, works and wonders that are more significant in the non-existence, their disappearance and their shadow than in being there." He is tantalising, too, on another class of vanished work: Welles's theatre productions, especially his voodoo Macbeth and his Doctor Faustus.
But Kane, which Thomson reads as a portrait and a prophecy of the artist's own life, haunts and bullies the book, and in some measure even determines its structure: recall - the author certainly does - that the investigator who unravels the film's plot is called Thompson. Rosebud begins with the lonesome death of Citizen Welles in 1985, and with his last words as we know them, a message on the answering machine of the director Henry Jaglom, who was born the year Kane was released: "This is your friend. Don't forget to tell me how your mother is." If it's the case that he never uttered a syllable again, how apt that he should have died still harping on mothers, and on friendship. Thomson's orphan Welles is a man who passionately needed to be betrayed, as a means of being superior and alone.
The book ends with a trip Thomson took on the day of Welles's death to Nepenthe, a place on the Big Sur coast that Welles had picked out to be his hideaway with Rita Hayworth. The final dialogue with his imaginary publisher recapitulates the book's major themes: Welles as magnificent piece of work and poor bastard, at once deeply courageous and unfathomably self-absorbed. But its sombreness reaches beyond the man to enfold both Welles's chosen medium and his latest biographer. Welles made the greatest film of all time, yet the medium of cinema, Thomson has clearly come to believe in recent years, is horribly shallow compared to literature or painting or music. What does it mean for a man of - the word is carefully weighed - genius to spend his life pursuing such a bauble of an art?
And Thomson himself, whose soul was possessed by Welles when he saw Citizen Kane at 14 in the Tooting Classic, and who has now spent all his professional years writing about flickering images? Has he, too, squandered his life and talents on a trivial pursuit? The chill, stoical last words of Rosebud - its own "Rosebud", so to speak - are weirdly reminiscent of a great artist (greater than Welles) for whom, one suspects, Thomson may not greatly care - Samuel Beckett: "One has to do something."
Do the publishers require a shrieking blurb for Rosebud? Glad to oblige. Let them simply recycle, with due attention to the adjective's roots in "terror", the poster slogan for Citizen Kane: "It's terrific!".