Consciousness and the Brain (by Stanislas DeHaene, the mathematician-turned-neuroscientist whose other books taught me so much about how our brains process numbers) surveys a lot of research that has been done on the empirical phenomenon of consciousness. A typical experiment is to flash words on a screen so quickly that subjects don't notice them, or just slowly enough that they get noticed, and scan the subjects' brain in each case. It turns out that a huge amount of processing gets done below the threshold of conscious perception, but the processing stays within only a few brain regions. Above the consciousness threshold though the whole brain starts to resonate with the idea, putting it into a condensed form that can be stored within working memory.
This is far from the last word on the subject, but it's exciting to see consciousness move from the realm of gibberish quackery to the point where we can answer substantive questions about it. Another person who seems to be worth reading (DeHaene speaks highly of him, but I haven't followed up myself) is the philosopher Daniel Dennett.