Open Source Flexbooks

www.ck12.org has free open source STEM textbooks to download, based on state standards. They have dozens of free online books, which you can pick and choose chapters from to create your own digital textbook. You can also edit the chapters in an online editor, and add images and interactive applications (with HTML code) from online. I would recommend editing with a laptop or desktop, I couldn't get into the editor on my iPad. This site looks great for math and science teachers who wish to create their own textbooks. Textbooks can be published as PDFs and by creating a link to be viewed online.