Booklid
Good e-library for downloading and searching for books
5 Alternatives To Booklid
BookFI
Bookfi is one of the most popular Multi-lingual online libraries in the world.
Library Genesis
Library Genesis is a scientific community targeting collection of books on natural science disciplines and engineering.
Niigu
Head2Head magnet search engine alternative to the discontinued BTDB.TO
SCI-HUB
It provides mass and public access to tens of millions of research papers
arXiv
arXiv is a free distribution service and an open-access archive for scholarly articles.