I've been starting a textbook project myself recently (it's a lot more work than I first anticipated to flesh out the material and formatting, even with latex, and I haven't even started with the graphic design) so this is quite interesting to me. I saw that ibooks author will export to pdf, my biggest question is: will this be compatible with print-on-demand services? Surely some folks are still going to want dead tree versions, and some topics don't need lots of media interaction. Apple would do very well to consider hosting their own print-on-demand services for this (hmm, sort of like they did with the cards iphone app...)
Another thing I would love to see is some sort of "preview this book" like Amazon's. If Apple is to be believed, this is going to open up book publishing quite a bit, which also means lots of competing books with the standard web-2.0 and appstore quality distribution.