Books
Google Books
Google Books is open access, meaning login is not required.
Millions of scanned books, but usually only partial copies.
Some books are more appropriate for academic study than others, therefore check:
Who are the authors: are they researchers, university tutors, or experienced professionals/practitioners? Does the book have endorsements from other researchers?
For whom it is written: various books are written for different audiences, e.g. general public, undergraduate or postgraduate students, and specialists in a particular subject.
Who is the publisher: have they published other academic titles? If the publisher is obscure or unknown, check again for reputable authors and endorsements.
When was the book published: some titles in Google Books are not the latest editions.