I’m torn. This is like saying how can one possibly cut down a beautiful tree to make bathroom tissue, but we still need to get toilet paper from somewhere. Not to mention that what I believe to be a crummy book may be someone else’s all-time favorite read. My suggestion would therefore be to use brand new books. There’s plenty more where they came from; you’re helping a (possibly) struggling author earn his/her royalties; and there is little or no chance of destroying even inadvertently a long out-of-print work.