I've been to AC 3 times already, but still like to get the latest info. I just read Lan Sluder's San Pedro Cool travel guide from front to back, and think it's a great resource. It focuses on Ambergris, Caye Caulker and the activities available within and from those locations. For those like me who plan to stay on the island their whole vacation, that's all you'll need. Between this guide, the Green Guide and the suggestions that I know I will get from folks when I get to AC, I'm covered. I can leave the heavier, more general guide books behind.
Check this out... you'll like it.