I have finished the first chapter of my algebra book from the site www.ck12.org. ck-12 is a non profit organization based in Palo Alto that is dedicated to providing open source textbooks for a variety of subjects. They provide prewritten concept modules that are easy to edit and combine in any way you want. The books are available to students online, they can make a pdf or they can export to Kindle or iPad. It seems like a dream come true to me, they have a lot of momentum right now.