iTunes 9.1 - Books
Following Kirk's tip, I downloaded a version of P.G. Wodehouse's "Right Ho, Jeeves" from Gutenberg, added it to iTunes, and did some 'script to it.
It behaves like a track in that it has most of the same properties, can be copied to playlists, and so on. The track kind of a track-that-is-a book is "Book", at least my Wodehouse book's is "Book". Properties that don't apply to "bookness", such as start and finish, time, and so on, return zero or null values. Still, some operations you would perform on a music track will error when you run them on a book track, eg: setting the lyrics property.
Books can have artwork. Andperhaps obviouslyBooks can't be copied to an iPod/iPhone.
More as it develops.
[UPDATE: the kind of a book track may actually vary depending on the, uh, kind of book. That is, if iTunes "knows" it's a book from iBooks, then the kind may be "Purchased iBook" for all I know.]