CK-12 Foundation's FlexBook tool allows teachers to develop their own multimedia textbooks. Those books can be shared publicly with the CK-12 community. This CK-12 released a list of the most used FlexBooks of the year.
This year's five most used CK-12 FlexBooks are Biology Concepts, Chemistry Concepts, Life Science Concepts for Middle School, Physical Science Concepts for Middle School, and Earth Science Concepts for Middle School.
If you would like to try developing your own FlexBooks, the tutorials embedded below will help you get started.