I think it depends on the kids you have. Last year I had this terror of a two year old boy who destroyed everything in his path, books included. It got to the point where I had to shadow him the entire day, but that's a whole other story.
Right now I've got 4 little ones who play very gently with things, so I've got not only board books, but regular paperback books out that they can look at whenever.
If the kids can't respect your things, then they shouldn't be trusted with them unsupervised. While you teach them how to respect books, maybe they should be put up until a structured story/reading time. Then you can have a pile of books on the floor, they pick out 3 and then go find a nice place (where you can see them, of course) to look at the books. This way they are able to explore books but in a more structured setting.
They can always be reinstated once the boys become accustomed to treating things with respect.