https://alternativeto.net/software/z-lib/ (article listing alts for z-library, though z-lib is a great website and you don't need alts unless you've already maxed out of your 10 free books that day haha)
https://www.tckpublishing.com/websites-download-free-ebooks/ (another article list of 29 best websites)
and just as a Christmas bonus, I'm throwing in an article about how to keep your ebooks longer than you checked them out for: https://bookriot.com/how-to-keep-library-ebooks-longer/ as well as an article about how to best use libby, the free library app: https://timeinthemarket.com/libby-the-best-way-to-access-free-audio-books-and-ebooks/