Aaron,
I got Enter the Zone from Amazon.com, but really all the info you need you can get in Crossfit Journal #21 for $5 US in PDF format. Order it off the main page at
www.crossfit.com. It doesnt really go into the theory but it has all the nuts and bolts to do the zone. Alot of info can also be found at
www.zoneperfect.com. Click on 40/30/30 nutrition, then getting started, then mastering the zone to get some theory behind it.