Uncovered by research firm Busted Loop, via Forbes, among other data like 16,700 iPhone apps have been iPad-certified, is a list of possible ebook categories for the iBooks store:

Apple has designated about 20 "top-level" categories for books, including "Fiction & Literature", "Reference," "Romance," "Cookbooks" and "Comics & Graphic Novels." Below those categories lie more than 150 sub-categories, including some very specific genres, such as "Manga" under "Comics & Graphic Novels," "Special Ingredients" under "Cookbooks," and "Etiquette" under "Reference." Some sub-categories, such as "Fantasy" and "Science Fiction & Literature," even have sub-sub-categories ("Historical" and "Paranormal," for example.) There are also two sections for "Erotica" books; one under "Fiction & Literature" and one under "Romance."

I have the feeling it's not the tits kind of erotica, so much as Marquis de Sade, but we'll see. [Forbes via MacRumors]