Books, with rare exceptions such as that of K.J. Rowling's and Dan Brown's, are not a global phenomenon, draw less attention, making the crime of piracy less tempting or less lucrative. Books are more elitist and TV series more vulgar. That's also the reason why it's more profitable to harvest clicks on porn or pirated music than on metaphisical poets such as John Donne.
Since there are fewer book pirates it's also easier to track them vs. millions engaged in pirating movies on P2P networks.
My second guess would be that e-books are quite a new phenomenon (in terms of the recent surge in popularity of e-paper readers and tablets) compared with optical drives and tapes that were around for a couple of decades. DRM techniques were much less advanced and so piracy could flourish. The market and the music or film industries were so badly hit that they had to find a different business model to make ends meet.
Kindles and iPads are made with DRM in mind, the techniques are more advanced today, and the publishers have not been threatened by piracy so much as by the shrinking readership. Also scanning entire books could not have been done so massively and easily as copying music or films.
Finally - a word in favour of piracy - I can imagine, although it's just a guess, that it was exactly the piracy that drove evolution and innovation in film and music towards more spectacular entertainment, 3D, iMAX, 4D, live performance, stand-up shows, HD tv screens, VR goggles, 5.1 stereo, group gaming, motion sensors etc. etc., because watching flat, low res. series or films is not considered a proper fun anymore (even The X-files seems boring today, not engaging enough, from a different epoch, like ballet).