• American Institute of Mathematics 

A list of open textbooks in various subdisciplines of Mathematics approved by the AIM editorial board.

• BC OpenEd 

A curated collection of open textbooks, many reviewed by British Columbia faculty.

• Bloomsbury Academic 

Bloomsbury, a well-respected and long-time publisher, has released some of their academic titles for open access/open education.

• FreeBooks4Doctors 

Free medical textbooks.

• Lyryx Open Textbooks 

Lyryx offers high-quality open textbooks in the fields of accounting, mathematics, and economics. They also have optional added (paid) resources such as homework and quizzes to accompany the open textbooks.

• Noba - Psychology Modules 
  
  A series of modules in the field of Psychology from introductory courses to more advanced topics.

• OpenStax College 

Contains peer reviewed, quality open textbooks with interactive components. There are some amazing textbooks available here, with more in production.

• Open Textbook Library 

Hundreds of complete, open college-level textbooks collected by the University of Minnesota.

• Pressbooks Directory 

The open textbooks selection of the popular publishing platform Pressbooks. In addition to text and images, the textbooks on this platform often include videos and interactive elements.

• Project Gutenberg 

Find the full text of classics and public domain works from the first massive ebook creating organization in existence. Nothing fancy here, just files with the full text.

• WikiBooks 

A project of the Wikimedia Foundation, this collection of group written textbooks in a variety of sources follows rules similar to Wikipedia.

For more sources of open textbooks, check our extended resources list.