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.