Education
Modern Libraries
Note, the following may or may not have legal implications upon use; provided for informational and discussion purposes. Also, note that the direct links may change or be invalid due to the above. An internet search for the name of the site plus "proxy" may help if there are issues.
Library Genesis (wikipedia description)
Sci-Hub (wikipedia description)
Teaching and Learning Resources for Children
Honestly, most of these are not good; weird games and unnecessary flashiness instead of simple and direct access to books. If you find any good ones, please let me know.
Tumble Books - Online ONLY children's Books