OpenStax: Provides free, peer-reviewed textbooks covering various subjects such as math, science, social sciences, and humanities.

Wikibooks: Hosts a collection of open-content textbooks, manuals, and educational materials that are collaboratively edited and freely accessible.

Global Text Project: Offers open-content electronic textbooks freely available to anyone, with a focus on providing educational resources to underserved populations worldwide.

Project Gutenberg: Offers over 60,000 free eBooks, including many classics, available for download in various formats.

Bookboon: Offers free textbooks for students and professionals, sponsored by advertisers. Covers subjects such as business, engineering, IT, and more.

HathiTrust Digital Library: Offers millions of titles digitized from libraries worldwide, providing access to both public domain and copyrighted materials with full-text access to public domain works.

Bartleby.com: Provides a wide range of reference works, literature, and non-fiction books for free online reading, including classics and other educational resources.

OAPEN (Open Access Publishing in European Networks): Hosts a collection of open access academic books primarily in the humanities and social sciences from various publishers.

DOAB (Directory of Open Access Books): Provides access to academic, peer-reviewed books available under open access licenses, covering various disciplines.

Internet Archive: A non-profit library offering millions of free books, including rare and out-of-print titles, available for online reading or download.