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.
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.