What is a book plate?

A book plate (the personal library version) was highlighted on Rosina Lippi's blog, but here are some pictures of other plates:

As for Avery stickers, they are fine as long as they are something that bookseller wants to use. If I could get ahold of avery stickers with a lifetime supply of Gregory Maguire or Christopher Moore's autographs, I'd be alllllllll about those avery stickers.