The internet archive is good although it depends what books you are looking for. I'd recommend worldofbooks.com too! It's not free but they sell second hand books at a cheap price. Not only that but you can choose the condition of the book and they always have really good discounts available as well as free shipping.
I use Anna's Archive if I need to find free textbooks and things but they have fiction on there too! Though you may also need to get an epub reader as not all books are pdfs.