I remember spending hours in libraries when I was young. Wait- I still do.
Do you ever wonder what books end up there? Who chooses them? Why?
A library seems like a magical place to those who love words. Anything can be discovered within those walls, among the shelves. You only need to figure out where to look, and the librarian is a good person to help.
At least, the good librarians do. Sometimes we run across the ones who hold books more sacred than the bookworms – who refuse to let the solace of the library be disturbed for any reason. Luckily, those seem to be few and far between.
Makes me want to visit the library again soon.
Ransom Noble 
To answer part of your questions…cuz I seriously know random things (and sometimes it comes in handy to have a librarian for a mother):
In public libraries they have a librarian who’s job it is to order the books. They look through the best sellers, and keep track of what books go out most often, and also take requests from patrons. (Rocks when you know the librarian and she’s willing to order a book for you!)
In school libraries, its the head librarian who does it. In colleges–most often its the reference librarian.