Ripping my heart with the pages

B is usually good with books. He and P do get rowdy sometimes, throw books, bounce with them on the bed, etc, so lately I've taken to keeping them all in the other room when I'm babysitting (P is also less gentle with books). Usually if even a tiny tear occurs, B is handing it to me needing it fixed right now. But today (obviously no P influencing him, being Sunday and no babysitting), I heard a tearing noise from his room. I cringed and tried to think of what magazine or papers he might have in there to tear. Unable to think of any, I went to investigate. B was quite calmly, and deliberately, ripping out the pages in his once favorite book about cars. It made me sick to my stomach seeing the book in that condition. I didn't get mad at him. It's his book, his choice how to treat it. I calmly told him to put the pieces in the garbage. Which he did, showing no remorse at the loss of the book. It will be some time before I feel comfortable leaving the books in his room again. If he wants a story, he will have to pick one from the box in the front room, and be supervised the entire time. Then it goes right back into the box. It's just so sad that he treated his book that way. I just can't understand it.