HathiTrust has digital content one can search and view free of charge. Most of the content consists of historical texts and manuscripts that are in the Public Domain (materials not subject to copyright).
Internet Archive, like HathiTrust, specializes in Public Domain digital content. There is more downloadable content available through Internet Archive than in HathiTrust.
Creative Commons is a repository for a diverse assortment of online content with generous access and usage licenses.
DOAB (Directory of Open Access Books) is a directory of open access, academic books you can read online for free.
DOAJ (Directory of Open Access Journals) is a directory of open access, academic journals you can search and read online for free.
OpenStax provides free access to open access textbooks and educational resources.
Government Documents and Information include federal and state-produced statistics, books, reports, etc.
You can always search a wide range of scholarly content through Google Scholar.