My girls have always *enjoyed* their books like that too - so for that reason I always put special books away so they wouldnt get ruined.
Someone said to me once that trying to keep books "pretty" or "perfect" (ie unused!) doesn't help your kids to have a love of books, as it can be too hard, or too much work to enjoy playing with them/reading them.
She said that by letting them have access to them, and to let them "experience" and "enjoy" them is the best thing you can do
So, I relented, and now they are much better (books hardly ever get ripped or anything now!) and they also love reading! I quite often find the two girls cuddled up somewhere reading stories to one another