Library policies no doubt vary across the nation, (or even between counties within a state,) but there are basically two ways to get Esperanto books into a public library: Donate them straight out, or convince the library to acquire the desired books.
Before you donate a book, or books to a library, make your choices wisely. Does the library system you are donating to already have any books about Esperanto? If so, you may want to keep away from text books and stick to some supplemental readers.
You will want to talk to the librarian directly about donating books. Just dropping them off at the front desk is putting your valuable books on the track to the book sale, or worse to the trash bin.
I have found that getting a library to buy Esperanto books for their collection to be problematic. Most librarians don't realize that the Esperanto-USA book catalog exists and will look up any books in their resources which usually won't contain our books. If you have confidence that the library can locate the books for purchase, give them specific recommendations of books, and try to supply them with the title, author and ISBN number.
For donating to a library bereft of Esperanto books, Firstly, I recommend the book by David RICHARDSON, ESPERANTO - Learning and Using the International Language because it is modern and contains information, a textbook, and a reader. Secondly I recommend a dictionary: The Esperanto Dictionary by J.C. Wells (if you can find one.) Thirdly I recommend some kind of reader, such as Pasxoj al Plena Posedo. Peruse the catalog online or ask the Libroservo for suggestions which are in stock.