I find it fascinating (Ironic?) that iTunes is now seen as the "preferred platform for artists wishing to make money selling their music" - given both it's history, as well the complete and utter lack of DRM associated with iTunes Music.
DRM is annoying, but it can be effective. There is a DRM free copy of every popular science fiction novel you can think of up to around 2010 or so (And, since I haven't checked for about a year, we may have moved into 2011/2012).
But, with the exception of Top 100 novels - it's next to impossible to find/download these books unless you try really, really hard. I'm sure many of those people who buy a book on Amazon, would instead download if it were effortless to. (Though, many would also purchase it, in much the same way many of us purchase our TV shows and Music.)
DRM on books, so far, has slowed down piracy of books. It's not 100% effective, but it's certainly not 100% useless.
More likely a lot of this stuff is just less popular so there are going to be less people seeding it etc.
I'm going to guess that piracy has a very long tail and the majority of piracy traffic happens with a small minority of files.
There's also the issue that books are generally cheaper.
Books also have some advantage in that it is more difficult to exploit the analog gap there, you need a way to print it out and rescan it. Or take a photo of each page and then use OCR.
Music on the other hand can be pirated by plugging the output from your speaker jack into something else that records it.
