cK-12.org is a website that contains free digital textbooks for science and math courses. You can use the textbooks as is, or you can customize the books to your liking, incorporating text and pictures from various sources into one book. The resultant book can be used as an ebook, or it can be made into a pdf document, which could be printed or shared as you would any pdf file. The books are all open source, so you shouldn't run into copyright issues. You can read more about the policies in the Frequently Asked Questions.
In full disclosure, I have not spent a lot of time playing with the flexbooks, as I don't currently have need for a textbook. But, I thought it a worthy resource to pass along for teachers looking to supplement their current texts and parents looking for resources for their students, especially in an economic climate where new textbooks may drop a step or to on the priority list.
If you create your own flexbook (or have already done so), I'd love to hear about your experience and how valuable a tool you found this to be.