I'm reading The Origin of Consciousness in the Breakdown of the Bicameral Mind. It's a fascinating hypothesis, if somewhat outlandish. The text is a little dated, as far as the neurological literature is concerned, but so far only minor pieces of evidence are made moot by modern research, and I can't think of any good reason why the theory couldn't hold up.
Robert J. Sawyer used that book a lot in developing the story for his WWW
), in which the Internet gains sentience. I've considered finding a copy and checking it out, but my "to read" list is still way too long as it is.