We generally do NOT recommend you dump your textbooks at the library free shelves because they almost never get taken and it burdens staff with throwing out the books. You are better to simply arrange to recycle it yourself. 

Assessment books that are filled in should be recycled like scrap paper.

Note that few people are interested in outdated textbooks and encyclopedias. You can try give them away but please do not dump them on library free shelves which is just passing on the disposal burden to someone else.