I had the good fortune to attend and present at this years NSTA Conference in San Antonio, Texas. As I was walked through the gigantic vendor expo, I saw a small booth that was out of the way from the other big vendors exhibits. I recognized the company name from an Apple workshop I attended. It was CK.12.org. This organization has been working to put out quality FREE digital text books and now they have an iPad app. You can even modify the books to meet your specific needs. All you need to do is create an account and start playing.