One Langford project that's been on the back burner forever is The Book of Imaginary Books, a compilation of all those famous works that don't actually exist except in sf and fantasy. Apart from being sublimely pointless and unsaleable to any mainstream publisher, it seems a wonderful idea.
H.P. Lovecraft invented the most famous non-book, the "unspeakable Necronomicon of the mad Arab Abdul Alhazred", parodied by Terry Pratchett as the Necrotelecomnicon or Liber Paginarum Fulvarum (Book of Yellow Pages) by Achmed the Mad, who preferred to be called Achmed the I Just Get These Headaches.
Lovecraft and his pals came up with heaps more sinister grimoires that appeared in their and others' fiction, like the Pnakotic Manuscripts, Ludwig Prinz's De Vermis Mysteriis (Mysteries of the Worm), von Junzt's Unaussprechlichen Culten (Hard-to-Pronounce Cults), and Cultes des Goules by the Comte d'Erlette (a little joke on fellow-author August Derleth).
Then there was the evil playscript The King in Yellow in Robert Chambers's book of the same title, which like the Necronomicon tended to drive people mad a lot, and of which it was said: "The author shot himself after bringing forth this monstrosity, didn't he?" Most writers feel that way after finishing their first draft.
Much later, John Barnes's whimsical fantasy One for the Morning Glory condensed all possible manuals of the Dark Arts into two volumes: Highly Unpleasant Things It Is Sometimes Necessary To Know and Things It Is Not Good To Know At All.
There are many more nonexistent but rather jolly titles. An Anthony Boucher sf story features a future manual of simplified English grammar called, logically enough, This Bees Speech. Douglas Adams would have had to invent the Encyclopedia Galactica to jeer at, if Isaac Asimov hadn't already done so in his Foundation series. Philip K. Dick's alternate history The Man in the High Castle contains the novel The Grasshopper Lies Heavy by Hawthorne Abendson, describing yet another alternate world that isn't ours.
The colourful Demon Princes sf sequence by Jack Vance includes vitriolic reviews of the massive ten-volume philosophical work Life by Unspiek, Baron Bodissey – who has the last laugh, since in Vance novels set many centuries later Life has become a revered classic taught in schools, as one day will John Brosnan's columns. [Brosnan also wrote for SFX.] J.R.R. Tolkien claimed to have cribbed The Lord of the Rings from The Red Book of Westmarch, but is it plagiarism when you steal from a book you invented?
Some fantasies feature whole libraries of imaginary books, the most famous nowadays being the Library of Dream in Neil Gaiman's Sandman, containing every book anyone ever dreamed of writing – from G.K. Chesterton's The Man Who Was October (a sequel to his actual The Man Who Was Thursday?) to lots and lots of titles like The Best-selling Romantic Spy Thriller I used to think about on the bus ...
It's less well known that Gaiman is paying homage to one of fantasy's all-time greats, James Branch Cabell, whose 1919 book Beyond Life contains a dream library featuring the stories that authors intended to write (rather than what actually got published), plus books by fictional authors – The Complete Works of David Copperfield – and planned works that never got written at all: "Milton's own King Arthur, by the by, is quite his most readable performance."
But the best ever book collection, if only it were properly catalogued, was invented by Jorge Luis Borges for his short story "The Library of Babel". It includes everything, all editions of all books, written or unwritten. It has to, since its volumes comprise every possible book-length combination of the letters of the alphabet (with spaces and punctuation). The only snag is that – just as in web publishing – the good stuff is almost impossible to find amid endless screeds of rubbish.
Maybe Borges never actually calculated how many volumes his Library of Babel would need to contain, but I had a go and came up with a number whose length approached two million digits. A bit more calculation, and it turned out that if each book could somehow be printed on an individual electron, there still aren't enough electrons in our universe to hold the Library. (And the universe isn't large enough to hold the actual number of electrons needed.) Now that's big.
I think I need to lie down for a bit ...
David Langford just remembered that Max Beerbohm pinched his idea of writing about imaginary books back in 1914. You can't win.