4. If found, evaluate the source
5. Check license restrictions and apply where necessary
6. Adapt and don't forget to attribute the author (if applicable)
Chem LibreTexts - is a Chemistry Living Library is a principal hub of the LibreTexts project, which is a multi-institutional collaborative venture to develop the next generation of open-access texts to improve postsecondary education at all levels of higher learning. The LibreTexts approach is highly collaborative where an Open Access textbook environment is under constant revision by students, faculty, and outside experts to supplant conventional paper-based books.
NSDL: National Science Digital Library: This repository gathers all possible online open resources from various providers in all STEM disciplines. Resources are browsable by education level, resource type, and subject. Even though most resources are free on this site, there are also some that have some cost associated with them.
IntechOpen - Open Access Book - World’s largest Science, Technology & Medicine Open Access book publisher. The website contains more than 3,000 open access books distributed under CC BY 3.0 license in various subjects.