So far, there haven't been any more issues. This morning, I did catch Smokey sniffing at one of the shelves, but he smelled the pepper, kind of winced, and backed away from it. I figure the scat mat and pepper will work until I can get different bookshelves. I'm thinking of looking into either getting bookshelves with doors, which are fairly expensive or getting regular (but taller) shelves and keeping things off the bottom shelves.

As far as a cat tree goes, they do have one upstairs in the corner of the hallway. I'm wanting to get another (and bigger) one for in the one corner of the living room. Granted, they don't get on the shelves as they're too big to fit with the books, although they have gotten on top of the bookcases themselves.

I do have a feeling that it is most likely Smokey who is guilty since Casey has never shown any interest in the books and has never done anything as destructive as that.