The non-profit CK-12 Foundation offers free digital textbooks in the STEM areas of learning…that is, in science, technology, engineering, and mathematics subjects. CK-12 aligns with state standards. These digital textbooks are available for download on your computer, iPad, Kindle, or other compatible e-reader.
They also offer interactive learning – for example, you may create your own classroom or group, and assign your students lessons in math and science subjects.
Here’s a video explaining the CK-12 Study Guides better than I can:
All of the CK-12 are Kindle- or eReader- ready and available for immediate download at the Kindle store, here. Most of them are free!