The CK-12 Foundation have added to their list of free textbooks. I've included links below for the new titles on Kindle (6 new, 14 total), but I understand you can get these from iTunes, also, if you prefer that venue, as well as get them from the Foundation's website (some are online only or PDF, those that are more finalized are in EPUB or MOBI). These are quite large files (up to 100MB, >1,000 pages, for some), so you may want to transfer via USB at Amazon (or read them on your PC) and not try to get every volume at once on their website (or they'll be hammered for days on their bandwidth). With iTunes, I'd consider buying via the desktop app and transferring via USB, just to keep from timing out on a WiFi connection to your tablet or phone (or driving your cellular bill sky high if using it for the data transfer). I did check one book from Amazon - there was no DRM and I don't expect it on any of the others.