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. 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 is a video that explains the Flexbooks better than I can (their site is huge!):
CK-12 Flexbooks align with state standards. Visit CK-12 Flexbooks to browse the site to see if it’s a good fit for your homeschooling family (ie. if the standards align with you!). Create a classroom to see how that area works, then take a quiz (oh, my goodness, I did poorly on the algebra quiz I took!).