Levitin's book is a great way to get into it! Great book for the daily commute, pick up and read anytime.
Although, you can skip the first few chapters on music theory if you're already familiar with it... otherwise IMHO there are better sources for that too.
Here's another one I just started reading:
"The Music Instinct: How Music Works and Why We Can't Do Without It"
I'm only a few chapters in but it's very promising, and even the music theory chapters have some very interesting details I wasn't aware of.
Also, check out the research coming out of the BRAMS lab (http://brams.org/). I just toured their facilities the other week. Jaw-dropping. There are some amazing individuals doing very interesting research there.