If you need a book for a project, you might want to check out Project Gutenberg. They provide researchers and readers with open source ebooks in a variety of formats. While they have an extensive collection, they are limited by their desire to only publish works that are out of copyright (you can be fairly sure that anything you find on their US site is out of copyright in the US); so, you won’t find any recently published books here. However, their collection is really quite comprehensive, and they often have titles that are difficult (or expensive) to find anywhere else. Their search function works well, but you may find it easier to do an internet search (in your favorite search engine) starting with Project Gutenberg and including the topic that you are looking for. One other word of caution–if you need something for a research paper, make sure you know if you are looking at a fiction or a non-fiction book. Sometimes that is not make completely clear on the site.