Which are the best books that never existed? asks David Barnett at the Guardian blog.
The obvious answer aside, however, many are suggested in the article and comments of which I've never heard, and a few of which I have (eg the Hitchhiker's Guide to the Galaxy which seems to be a Guardian readers' favourite.) Here are a few from the comments: Jason King's great "Mark Caine" adventures, including The Lady Is Ready; Frank Bascombe's collection of short stories and also Nathan Zuckerman's Carnovsky (would it be better than Portnoy's Complaint, I wonder?); The Life and Letters of Silenus, Nymphs and Their Ways, Men, Monks and Gamekeepers: A Study in Popular Legend, and Is Man a Myth? are all on Mr Tumnus's shelves in The Lion, The Witch and The Wardrobe; and Camel Ride to the Tomb, by X. Trapnel, a character in Anthony Powell's A Dance to the Music of Time and The Doubtful Asphodel by Sebastian Knight, from Nabokov's The Real Life of Sebastian Knight, are two non-existent books I'd like to read, but Stanislaw Lem wrote a whole marvellous volume – A Perfect Vacuum – of reviews of books which did not, and in some cases could never, exist. There are plenty more suggestions, including the inevitable few from people who haven't (yet) got their own works published.
Apart from Hitchhiker's, David Barnett's choices (links at Guardian post) are: Necromonicon (H. P. Lovecraft); the Ficciones of Jorge Luis Borges; books in the library described in The Abortion: a historical romance 1966 by Richard Brautigan; The Pension Grillparzer from The World According to Garp (I thought John Irving pretty much did write that in reality, in a subsequent novel); and The Blind Assassin by 'Laura Chase' (Margaret Atwood).