A colleague brought humblebundle to our attention, and since there are many questions about where to get learning materials like books. These are limited time offers, so feel free to check to see if they have anything else robotic or programming related. RobotShop is not affiliated with this site, though their approach of supporting charity via these purchases is a good one. At the time of this post, these books are on offer:
Know any other sources for free or greatly discounted learning material?