Flexbooks for Instruction - Secondary
The CK-12 project has added some new titles. Although most of the material is for High School, there are some Gr6 and 7 Math texts.
"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."
Use Quickstart to build your own: