I just downloaded the Inkling app from the iTunes Apps Store
. It allows you to buy interactive textbooks for the iPad. I downloaded a free chapter of this Music book
to sample. I can't say too much about it yet, as I won't be exploring that sample for a couple of days, but it looks quite interesting. The books aren't cheap, however, but you can buy them a chapter at a time (although you'll pay more in the long run that way).