Following the launch of iBooks 2 and the introduction of textbooks to the iBookstore, Apple has pushed out iTunes update 10.5.3 with support for textbook syncing to the iPad.
About iTunes 10.5.3
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.
It looks like this update is necessary for textbook syncing, so if you’ve been struggling to install new textbooks on your iPad since they launched this morning, you might want to install this and try again.