Awesome! I really hope this gains momentum; and I'm sure textbook publishers hope it doesn't. What a brilliant idea: pay renowned authors & professionals in their fields to write a high quality textbook, and publish it for free to students. Supported by the Bill & Melinda Gates Foundation, William & Flora Hewlitt Foundation, Twenty Million Minds Foundation, etc. This is to textbooks like Khan Academy is to instruction.
Only a handful of text available so far, but they seem to be of pretty good quality, imho. The physics has 1269 pages of awesomey goodness.