Another source of free books is libraries.
If you have a local library, ask them about ebooks. Most libraries can give you login information to your regional ebook library.
If you don't mind reading scanned book images (best with a phone or tablet), you can also use the
Internet Archive. They have many titles that have never had proper ebook editions. They hold the physical books in storage, and digitally lend them out to one reader at a time.