It sounds like to me that he has a need of some sort. I think you need to get to the bottom of what that need IS. Demanding that he not do it isn't going to take away the underlying problem, and I think you should try to figure out what that problem is.
I would talk to your ped and ask for advice. Perhaps he or she would have some ideas for things you can allow your child to do to take the place of tearing books would help "work out" this issue with them.
When my nephew was around 3 years old, he would cuddle with his parents, grandparents and other adults. When he cuddled, he would "pinch" their arms. It wasn't a mean pinch - it was a way of comforting himself. My MIL let him because when she asked him why he was pinching, he said "I need to pinch." (Of course, the next day she had a bruise the size of an orange, and after that, she didn't let him pinch anymore, LOL!)
The thing is, his mother realized that fussing at him about the pinching wasn't fixing the problem. He had a need she didn't understand, so we talked to the ped about it. He asked her several questions, and there were other things going on that between them AND the pinching, pointed to having some sessions with an occupational therapist.
It really helped. A LOT.
So, I think that you should consider thinking along these lines. I would really consider this if I were you.
Dawn