Introductory college texts tend to be expensive, bloated and math-averse, from what I've seen. They have a little too much of the pretty coffee-table book in them. That said, a used copy of a major college text like "Understanding Earth" by Press and Siever would set you up well for whatever interests you most, whether it's paleontology, geophysics or simply the regional geology of your area.
The only textbook I ever refer to myself these days, though, is best suited for the solo reader with the desire to understand in depth--a rather old-fashioned kind of book. It's "Planet Earth: Cosmology, Geology, and the Evolution of Life and Environment" by Cesare Emiliani (Cambridge Press, 1992), a rigorous masterpiece that spills over the narrow boundaries of geology into physics, chemistry and biology as it should. It will be useful for many decades to come because, among other things, it doesn't shy away from the mathematics you need. It will teach you science, not just Earth science.
"Planet Earth" was conceived as an undergraduate text and is very personable in tone, but it seems to have more material than a one-year course can realistically present. However, a committed reader can absorb much faster than a professor can teach. The author died shortly after publication--he was a great scientist and communicator--so it probably will not be updated, and that's a shame. But that also makes copies a great bargain, costing a fraction of glossy texts weighing twice as much.
A book well suited for the humanities major--someone more at home with descriptions rather than equations--is "Earth: An Intimate History" by Richard Fortey (Random House, 2004). Fortey is a paleontologist, but he brings a Englishman's broad education to the subject and well expresses geology's poetry.
I review many other interesting possibilities in my Book Reviews list.
