As it stands, the humor book genre is ill-defined. Go into the humor section of a book store (or browse Amazon) and you are likely to find joke books beside books written by comedians next to humorous works of fiction. For some reason (Perhaps a lack of demand, perhaps the inevitable confusion caused by similar terminology) the genre has resisted stratification. Until now. The list below represents an attempt to define the various subgenres that traditionally fall under the humor umbrella.
General Humor Books
Typically, humor books are those designed around a central humorous concept, such as joke books. Modern examples include F is for Fail by Richard Benson, in which the humor derives from comments written by students and their teachers on various assignments, Letters from a Nut by Ted L. Nancy in which the Nancy author/character writes hilarious letters to companies and publishes their responses, and The Internet is a Playground: Irreverent Correspondences of an Evil Online Genius by David Thorne.
Books About Humor
These are (you guessed it) books about comedy and comedy writing. Great books in this subgenre include Poking a Dead Frog: Conversations with Today's Top Comedy Writers by Mike Sacks, Live From New York: The Complete, Uncensored History of Saturday Night Live as Told by Its Stars, Writers and Guests by James Andrew Miller and Tom Shales, and Sick in the Head: Conversations About Life in Comedy by Judd Apatow.
Books by Comedians
This one is also pretty obvious. Books in this category were written by professional comedians. Examples include Dad is Fat by Jim Gafigan. The Bedwetter: Stories of Courage, Redemption, and Pee by Sarah Silverman, The Girl with the Lower Back Tattoo by Amy Schumer. Bossy Pants by Tina Fey. Why Not Me? by Mindy Kaling, Yes Please by Amy Poehler, and Born Standing Up by Steve Martin.
Funny Essays or Memoirs:
Another obvious one. These writers employ their comedic talents while telling their life stories. Me Talk Pretty One Day by David Sedaris, Running with Scissors by Augusten Burroughs, and Let's Pretend This Never Happened by Jenny Lawson.
These are serious novels that also happen to be very funny. Confederacy of Dunces by John Kennedy Toole, The Hitchhiker's Guide to the Galaxy series by Douglas Adams, The Princess Bride by William Goldman. Classic examples include the books of PG Wodehouse and Three Men in a Boat by Jerome K. Jerome.
Here's where it gets a little tricky. In these books, humor supersedes story. John Swartzwelder's The Time Machine Did It isn't likely to satisfy fans of mystery or science fiction, but diehard fans of The Simpsons will love it. Jack Handey's The Stench of Honolulu is another great example.
Did I leave out a category? Let me know in the comments, and feel free to recommend your favorite funny books.