Google Scholar filters out everything except the scholarly resources that you need. You can access Google Scholar here. There is a guide on Google Scholar here.
HathiTrust is a partnership of academic & research institutions, offering a collection of millions of titles digitized from libraries around the world. Searchable through TechCat as well as the HathiTrust website.
Internet Archive hosts millions of free items (books, movies, software, music, websites, and more) in the public domain or written under a Creative Commons license. You can access the Internet Archive here.
Project Gutenberg offers over 54,000 eBooks in the public domain for free. It's mission is to digitize and archive important cultural works. Anybody can access this collection here.