Two books by the same author, Alan Hobson, take your pick
Consciousness - more of a textbook, for people already familiar with biology, in particular the nervous system & brain
The Chemistry of the Conscious State - more for lay people, doesn't require prior knowledge

the odd thing is the names of these books should be switched, IMO, but in any event ....

I know there are some well read intelligent people here, if you read these I'd be curious to hear what you think. Answers the BIG question. (IMO)

Now that I have the answer, time to go paddlin'