Abigail Goben posted about how she decides which books to buy for her library. She says she reads reviews, listens to others in her field, and keeps herself well connected through the interwebs.
She of course forgot to mention the true decider: witchcraft. Dum dum duuummmm.
...She also explains what you can do as an author to not creep out librarians and get them to buy your book. Victory, information!