Please note, Libgen and Z-Library have books that are copyrighted and ones that aren’t. The Internet Archive sometimes has copyrighted works, but often, it does not. I am not suggesting or promoting the download of copyrighted works. This website only makes you aware of their existence on such sites so that you may download them when they enter the public domain or download today the works that already exist in the public domain.
With that said, I suggest that you use Z-Library or the Internet Archive for your perusal of books, as they have the best user interface. If the book you are looking for is not available there, then Libgen may have it. Some people like Project Gutenberg, but I dislike the UI and find that the Internet Archive has everything Project Gutenberg has but looks better.
Z-Library (not to be confused with The Zerwic Library)
Libgen
And other mirrors
The Internet Archive
Project Gutenberg