Ethel the Aardvark Goes Quantity Surveying , and about fifty more from that marvelous Python sketch.
Not the The Princess Bride as written by S. Morganstern?
My hands-down favorite is The Unstrung Harp from The Unstrung Harp: Or, Mr. Earbrass Writes A Novel by Edward Gorey. (“The Unstrung Harp” is the name both of Mr. Gorey’s real book, and his character Mr. Earbrass’s novel, which would be the “fake” book.) The story of Mr. Earbrass’s saga to write a novel is endlessly amusing, especially with Gorey’s signature illustrations. It’s a book that’s close to the heart of many an aspiring author.
If I could get a first edition of The Unstrung Harp, I would be sublimely happy.
De Selby’s metaphysical blockbuster Golden Hours, from Flann O’Brien’s The Third Policeman.
Jodi: GREAT one. For the hell of it, I searched the Godternet for a first addition, just to see what prices came up. I’m sure with more research, you could find others, but AbeBooks seems to have all manner of options from $150 - $1000.
You made me want to go home & pull down Amphigorey & reread this. Let’s move it along now, work-day.
sigh
I really, really thought that the thread title said “FAKE BOOBS.” I’m not sure if it’s a case of being too tired to browse or simply wishful thinking.
Well, since I’m here, I may as well add my favorite (next to Hitchhiker’s) fake: The Nice and Accurate Prophecies of Agnes Nutter, Witch.
Grate Expectations by Charles Dikkens, the well known Dutch author.
Don’t you mean Achmed the I Just Get These Headaches? 
The Book of Night with Moon; a fictional book in the Young Wizards universe, which is also the title of a real book set in that universe.
Sternenkrieg by Gustav Anderman and Tenets of war by Tanakov, both treatises on warfare in the Honor Harrington universe.
The Book of Silence, book of the God of Time and creator of the universe in the Lords of Dus series; to speak a single word from that book is death.
I’ve read this. Of course I read the one by Leslie Groves not Kilgore Trout.
You got the correct title though, which I missed.
The ‘Encyclopedia of the Marvels, Life Forms and Other Phenomena of Zamonia and its Environs’ by Professor Abdullah Nightingale from The 13 1/2 Lives of Captain Bluebear is lovely…if anyone’s familiar with it…
Diuretics - Repo Man
Kid Shilleen and the Murder at Whiskey Gulch - Cat Ballou
Dixon Hill in the Big Goodbye, Dixon Hill in the Long Dark Tunnel - TNG
The Simpsons:
The Big Book of British Smiles
How To Cook For Forty Humans
Your Gimmicky Restaurant by Bennigan and Fudrucker
The Theory And Practice Of Oligarchical Collectivism (political treatise)
“The Cryptonomicon” is a fictional book in Neal Stephanson’s real book, “The Cryptonomicon”.
From Gilligan’s Island
Four Masted Schooners I have Known
A Million ways to make a Million
How to Tell A Mushroom From a Toadstool by the late Dr. Morton Kepstone
Hotel Royale by Todd Matthews.
Emperor of the Asteroids
The Builders of Mars
Fight for the Stars
The Twilight of Terra
Savior from Space
The Master Race
The Thousand Year Rule
The Triumph of the Will
Tomorrow the World
All by the noted science fiction writer Adolph Hitler.
The Omega Glory, from “Star Trek: Next Generation” (I think it shows up in a couple of other Star Trek series, too.
Quiddich Through the Ages, from the Harry Potter series. Oh, and there are also all of Harry’s textbooks, Mrs. Weasley’s recipe book, etc.
Are you all forgetting Oolon Coluphid’s trilogy of philosophical blockbusters: Where God Went Wrong, Some More Of God’s Greatest Mistakes, and Who Is This God Person Anyway?
Would those be by Stuart Best?