With bonus points for Zombies?
Several people made this point, but I don’t think it’s valid.
A book is a method of packaging printed material, with individual pages bound between covers. It’s not a description of the form of the contents, which might be a play, a novel, poetry, non-fiction, or photographs.
Shakespeare’s plays have all been published as books. Many of them were even published as individual books while he was alive.
I say they count.
Neil Simon and Paddy Chayefsky have to be up there (I don’t make a huge distinction between plays and books in this contexts, though I draw the line at teleplays). With Simon, it’s a little unclear how many source plays were adapted, since several languages are involved, and third-hand inspirations from The Odd Couple and The Sunshine Boys really gum up the count.
Ask again in ten years, and I bet Neil Gaiman and JK Rowling will be contenders.
Edgar Rice Burroughs’ characters pop up a lot and he wrote around 80 novels, but as far as I can tell, only twelve of his books have been directly adapted to film:
Tarzan of the Apes
A Princess of Mars
The People That Time Forgot
The Land that Time Forgot (I can’t swear that these two aren’t the same book)
At the Earth’s Core
Jungle Girl
The Lad and The Lion
Tarzan and the Jewels of Opar
Jungle Tales of Tarzan
The Return of Tarzan
The Son of Tarzan
The Oakdale Affair
Comic books aren’t being addressed, and except for origin stories, superhero comics should probably be excluded. (Sorry, Stan!) There is one cartoonist, though, whose stories were more story-focused, and whose comics were adapted more-or-less faithfully into 50 (low budget) films: Francisco Coching, the founder of modern Filipino comics!
The original interpretation of I Am Legend was vampires, and the author wasn’t (un)dead in 2003, but I am in awe of seeing Richard Matheson’s name on so many disparate properties. Lots of horror, sure, but also stuff like Real Steel (originally Steel) and TV shows.