An unpublished, self-published or otherwise distributed book that doesn't have an ISBN number but still resembles a traditionally published book. For example, if I've read Andy Weir's The Martian from his website, before it was officially published, it would still count.
Screenplays
An audiobook or radio play version of a published book, either unabridged or abridged or adapted as long as it's still clearly based on the book. For example, I recently listened to a Soviet radio adaptation of Jules Verne's 20000 leagues under the sea, and decided to include it despite it being heavily abridged and taking a lot of creative liberties.
Short stories, even those always published together, count as one entry each as long as each story can be read as a complete narrative. For example, Mikhail Bulgakov's A Young Doctor's Notebook is a collection of 6 to 8 stories (dependending on who you ask) of which I've read 7.
A text book and its associated audiobook count as one entry, not two, given that they're essentially the same book, just in different mediums.
What doesn't count as a book
Comics, both traditionally printed (for example, Art Spiegelman's Maus) and webcomics (for example, Sarah Welch's Signs of Three)
Fanfics, even ones as long as a book (for example, ErinPtah's He Says He Is An Experimental Theologian. This is not because I do not appreciate comic books or fanfics - quite on the contrary, it is precisely because I feel they are too unique to be included in this list. I read fanfics and comics for different reasons and with a different mindset that I do books. Perhaps in the future they'll get list pages of their own.
Blog posts and articles, again because the format is too different.
Podcasts and radio plays NOT based on existing books. The Welcome to Night Vale podcast doesn't count, but the Welcome to Night Vale novel, or its audiobook version narrated by Cecil Baldwin, does.