Free and Open-Source Learnin’s
Open Text Book is a useful blog devoted to free (as in speech, and also as in beer) academic texts. There’s only one post filed under “Geology”, which lists two books published by the USGS – one on ground-water hydrology, the other a basic introduction to plate tectonics. The math sections are a little beefier.
I should also mention that lectures from the Earth Will Kill Us All 101 course I’m teaching-assisting this semester are freely available as podcasts. Some of the students are using these for review, which is why they’re there, but we’re clearly getting a significant number of outside listeners as well. It gives me the warm fuzzies, though I do wonder who has time for this sort of thing (people with long commutes?).