The Internet Archive is a "digital library". It hosts a truly staggering quantity of content, including archived web pages and scans of many books. I've downloaded a few of their books to read on my Kindle. There seems to be some crossover with Project Gutenberg, but they have some books that Project Gutenberg don't have.
Calibre is an e-book library manager, viewer and format conversion tool. If you read e-books (whether on a Kindle or something else), I heartily recommend it.
Sigil is an ebook editor. It creates ePub format books, which my Kindle can't read, but Calibre makes it easy to convert ePub to Mobi (the Kindle's native format), and I've found that the simplest way to create books for my Kindle is to use Sigil to create an ePub, then use Calibre to convert the file and copy it onto my Kindle.