Downloading books

1. http://gen.lib.rus.ec/

2. https://b-ok.cc/

The second one has a faster download whereas the first has a webpage design that I like more, (it is simpler)
Learn to use these two websites. You will find most books on them. Try the other one if you can’t find a book on the one you would normally use.
My experience is that most of textbooks, best-selling books are available.

Downloading academic papers

3. https://sci-hub.tw/

You can also download majority of the academic paper using this tool.