In View Options for Books, the "Genre" column has been removed, and the "Artist" column appears to be renamed as "Author". In AppleScript it looks like the "Author" is accessible as a book track's artist. Also, FWIW, I'm not seeing my "Jeeves" ePub book in the XML.
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.]
Kirk confirms a Books folder in the iTunes Media folder. He explains how he actually got an ePub book into his library.
Apple has released iTunes 9.1 just in time for your iPad's arrival on Saturday.
Looks like AppleScript Application Bundles are still not displaying in the iTunes Script menu. I'm wondering now if this is permanent. (They still display in the system-wide Script Menu when placed in (or aliased to) your ~/Library/Scripts/Applications/iTunes/ folder.)
The "Audiobooks" master library is now "Books" and its special kind is changed from Audiobooks to Books. AppleScripts that use the old Audiobooks special kind will need to be updated to use Books in iTunes 9.1 and better. However, it looks like your audiobook files will remain in a "Audiobooks" folder for those of us using iTunes Media organization. Presumably, eBooks will go in a separate folder--"Books"? "eBooks"? "iBooks"? Our first eBook download for the iPad and eventual iTunes sync will tell the tale.
So far, this is the only change I've seen. More as it develops
[UPDATE: After a re-start, AppleScript app bundles were back in the Script menu!]