There are a variety of criteria I use. I try to get my schoolbooks posted quickly because those can be $50 or more each and I want to recoup my money before a new version comes out. I like to work through Amazon vine books because I explicitly asked for them.
Many of the books that are left in the music room are books sent to me by publishers that I'm interested in the topic but hadn't necessarily asked for them. Or books that someone suggested I'd like, and I agree and bought them, but haven't gotten to them yet.