Check out this LibGuide to find sites for free e-Books and navigate the world of formats and e-reader applications.
A word of caution...
Because of U.S. Copyright law, materials that are less than 70 years old are protected by copyright and are not free. Consequently, most books that you will find on these sites will be older than 70 years old.
There are still a lot of good books out there written more than 70 years ago - especially the classics! If you're taking an English or American literature class, maybe you can skip buying the book and read an e-book for free.