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The CK-12 Foundation:

“….is a non-profit organization with a mission to reduce the cost of textbook materials for the K-12 market both in the U.S. and worldwide. Using an open-content, web-based collaborative model termed the “FlexBook,” CK-12 intends to pioneer the generation and distribution of high quality educational content that will serve both as core text as well as provide an adaptive environment for learning through the FlexBook Platform.”

These are designed for the school market, but can equally cover first year undergraduate as well. They can be read online (Flexbook) or downloaded as a PDF. Amazon have also transferred them to Kindle format as well, such as the Earth Sciences text. A worthy resource.