Boundless is a service that provides free, open source, digital textbooks. There are 18 open textbooks with Creative Commons licensed content, for college subjects including accounting, biology, economics and more.
Boundless provides a free alternative to expensive textbooks and they can also be used as supplemental materials for students. The content is changed and additions made, in direct contrast to printed textbooks that are usually outdated before they are printed.
Boundless has an interesting process. They find the best free online content, have experts curate and vet it, and then deliver it in a way that is easy to read and navigate.
They also have some great features like SmartNotes which condense the full book into the main points, terms and examples, and Interactive Notebook to highlight items and add your own notes, Flash Cards, Quizzes, Study Guides and search.
Boundless is a great resource for college students, as well as advanced high school students and teachers. I'm a huge proponent of free resources vs. expensive textbooks and really like the fact that these free digital resources are constantly updated and improved.
Here are all of their textbooks:
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