Your book request letters should include any specific instructions for mailing donations. Also send a thank you letter to the donors when you receive the materials. Be sure to mention which books were the most helpful. If you get books you really can't use, it is good to let the donors know. This will help the donors to improve their programs in the future.
Some organisations complain that they never know if the books they send are truly helpful.
If the books are not what you need, explain the problem to the donors. It can be hard to tell someone who has given you a gift that there is a problem with it, but if done gently and politely, the donors will appreciate the information. Recruit people to help collect books for the library. Hold a book drive. Ask the organisations about book donations and other services for libraries.