Books are collection of a person's mind. Getting access to them is fundamental in our growth as humans. There have been several efforts to make it easier for humans to access all the knowledge of mankind for free.
One of the best projects that I love is Bookzz
eBooks
They refer to themselves as the world's largest eBook library. It'd have been true if not for more efforts by other folks.
eBooks, Journals and Science Books
Library Genesis has a superb collection of books, journals and scientific books. They're sometimes better than Bookzz. It neatly provides a list of multiple locations from which you can download the books. You can almost always find the ebooks you need.
It was created by the same community that created bookzz.org They claim to have a collection of over 50,000,000+ articles
Sci-hub is a clever system that gives you access to any journal that has a subscription from University subscriptions. That means, you can access any journal and any article that is available online. It's super easy to use. You just need to type in the DOI or the title of the article. Lo and behold! the article that was blocked for no reason is now yours. Go make the world a better place
This used to by my old favorite. It still has a few new books but the links are often dead and it leads to too many ads. I use it only as a last resort.
Other sources of docs
US Military Technical Documents
Most NASA Technical Docs
