I have been to the library many times, in many different capacities. I didn't ever wonder how the books got onto the shelves, but I did wonder who was choosing the books and how they chose them. Did the librarian go to Dymocks? Did they librarian have a massive Lucky Bookclub catalogue to peruse?
Selection Aids help teacher librarians to find out what resources are available for acquisition for the library. Promotions from suppliers, recommendations from colleagues and users, bibliographic services and reviews are all selection aids.
In some libraries, a representative from a book supplier or book publisher will come to the library with books and catalogues for the teacher librarian to look through and then choose whether they would like to buy any of the books. One advantage of this is that the teacher librarian gets to handle the book and take a good look inside. It can also be a disadvantage as the representative will be biased toward their own products regardless of their quality. The same could be said for viewing a publishers catalogue, although you don't get to touch the books.
Recommendations can often be very effective selection aids. If a resource is recommended, it is often done so with some authority, i.e someone who has used the book recommends it. Recommendations can also be in the form of advertisements, reviews and personal bias. Australian Standing Orders provides reliable, quality resources that meet requirements of the Australian Curriculum are used by many schools. ASO provide a number of books to the school every month for a flat rate. There are different packages of resources to choose from. My school uses ASO because of the quality of the books and their relevance to the learning curriculum.
Bibliographic services like SCiS can be relied upon for relevant resources and popularity. There are over 1.5 million resources on SCiS.
Reviews in journals, online reviews and social media are also effective selection aids when it comes to resources. If users don't rate a resource, they will let the public know. Social networking and colleague recommendations through discussions groups are also highly effective.
In combination with the library selection policy, this is how the books and other resources are chosen for a library.