Library Genesis Library Genesis (Libgen) : file-sharing website for scholarly journal articles, academic and general-interest books, images, comics, and magazines. http://gen.lib.rus.ec/
Z Library :is another free ebook library but it has download limit, if you don't create an account, you can only download 5 books per day. If you create an account then you can download 10 books per day. https://z-lib.org
Project Gutenberg :a library of over 60,000 free eBooks. Choose among free epub and Kindle eBooks, download them or read them online. https://www.gutenberg.org/
PDF Drive : a search engine for PDF files. The best part of PDF drive is that it doesn't have ads or download limit. https://www.pdfdrive.com/
BOOKFI : one of the most popular Multi-lingual online libraries in the world. It has more than 2230000 books. http://en.bookfi.net
College Students Text Books: Download a huge library of college textbooks for a very cheap price! https://collegestudenttextbook.org/
And also last but not least, Google!