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).