The library doesn't have to be the biggest branch in the area. In fact, the smaller libraries may be more grateful for your help. The books you help supply may be the only pro-gun books in that library. When students and other persons go to the library to research the gun issue, they'll find the carefully reasoned material that you laid out for them. At the simplest level, you can simply buy pro-rights books, and give them to the library.
Librarians strongly prefer hardback books, since they stand up so much better under heavy use.
Before putting down the money to buy the books for a donation, check with the librarian to make sure that the library would be interested in the book. Libraries may accept some of the books, and reject others. School libraries will probably want to review all of the offered books, to make sure that they are suitable for the relevant age group. Donations can also be done on a larger scale. The People's Rights Organization, of Columbus, Ohio, working with the national Citizen's Committee for the Right to Keep and Bear Arms, bought 25 copies each of three pro-gun books and donated them to the Columbus Library.