CK-12 provides free, customizable, digital textbooks. The one my daughter will use for Biology is even downloadable onto her Kindle, awesome. There is also a teachers edition that has lessons, worksheets, quizzes and tests. Online, the textbook is interactive, with videos and links. You have to contact them to get the full teachers edition with answers, but so worth it.

I've read about half the Honors Biology textbook myself and compared it to Biology, which I have and is often used in AP Biology courses and it is comparable. The graphics aren't as pretty, but I don't see this as a problem. Especially since I will be encouraging Callie to use many resources as she learns.

They also have other free textbooks in science, technology, engineering and math. I plan on also using their physics and chemistry in upcoming years.