Following iBooks 2.0, iBooks Author and iTunes U for iOS, Apple just released a new version of iTunes, 10.5.3. We're downloading the update to get our hands on a proper changelog but it's likely that this new version will simply bring support for textbooks and, perhaps, manual syncing of books created through iBooks Author.
Update: iTunes 10.5.3 does indeed enable textbook syncing to iPads.
iTunes 10.5.3 allows you to sync interactive iBooks textbooks to your iPad. These Multi-Touch textbooks are available for purchase from the iTunes Store on your Mac or from the iBookstore included with iBooks 2 on your iPad.
iBooks textbooks are created with iBooks Author — now available as a free download on the Mac App Store.
iTunes 10.5.3 should hit Software Update shortly; a direct download is available on Apple's website here.