I heartily recommend Julian Jaynes'
The Origins of Consciousness in the Breakdown of the Bicameral Mind.
The main theory presented in his book is revolutionary and suggests a fundamental reassessment of early human history.
Richard Dawkins remarked, "It is one of those books that is either complete rubbish or a work of consummate genius, nothing in between! Probably the former, but I'm hedging my bets."
Without trying to very clumsily sum up his theory, let's just say it has implications on the very nature of (still historical) early humanity, the cause/meaning of mental illness, and even the nature of God/religion among other things.
Let me encourage more articulate recommendations of the book to be also posted in this thread by other Jaynes enthusiasts.
- Ahi