VeryWellThumbed - Theory that goes like this:

If someone else's copy of a book whose principles you like is dirty, its corners are folded in several places and looks like it's been opened over and over again, you immediately can tell that the owner has read it passionately and the principles in that book have stuck.

Or at the very least you know you have an entire topic of conversation.

When you meet someone in this business, always check out the books at his/her desk. You'll learn a lot.

''Then again, there are some of us who cringe at having our books in less than meticulous condition. It's a lot harder to tell what we've been reading.''