I would have called music the least likely one to be making money, actually. A common usage pattern is buying one or two songs in a sitting, at which point transaction charges really become significant, and it has always seemed that the record labels take the lion's share of the payment.

iTunes TV, on the other hand, is probably more profitable, due to higher unit cost. Books almost _must_ be profitable, because they're in the same range as Kindle books, and Amazon wouldn't be pushing ebooks (and thus damaging their traditional book market) so hard if they couldn't make a profit on them.