For a useful guide to Tertiary level open textbooks by subject see OER by Discipline guide. (Search under 'Contents' on the top left hand side for discipline areas)
Unpaywall and Core Discovery are browser extensions for Firefox and Chrome, which add a green tab beside research articles that you can read for free.
The Open Access Button will search across the web for a freely accessible full-text version of an article and if not found will send an email to the author asking them to provide a copy