It's nothing new. There are a
ton of diet books out there based on the Glycemic Index.
The Montignac Method (republished & retitled as
The French Diet recently),
Sugar Busters!,
The New Glucose Revolution,
The GI Diet,
Good Carbs/Bad Carbs, just to name a few. Heck, even Somersizing & South Beach incorporate the Glycemic Index to an extent!
A lot of them you can find in used bookstores or as paperbacks new; probably a lot cheaper than the newest thing.
It works, in no small part, because it forces you to stop eating junk food. The GI isn't a magic system, IMO, but it
will help you feel full longer & therefore you'll wind up eating less over the course of a day. And if you keep the emphasis on whole foods, you'll wind up healthier as well. Do a Google search on the Glycemic Index; I know there's a searchable one online, but I'm not 100% sure where. Low GI foods are anything with a level of 50 or below.