I agree with what Murph said for the most part. I got Rosenberg's book a few months before heading down to Cozumel. I really enjoyed it - great tips on diving there, as well as restaurants, hotels, etc. My favorite part, though, was the pictures - some nice glossy shots of most of the usual suspects down there. Very good for flipping through when dreaming about your upcoming trip.
The info on the dive sites was variable. He talks about the depth, topography and current for almost every dive site. Some have sketches. Some are rated by experience level. Rosenberg's book is not the best in this regard.
For comprehensive info on the dive sites, I'd go with the book recommended above. I do not own it, but did get to flip through it on our dive boat and it looked extremely comprehensive from a dive planning perspective.
Actually, why not buy both? You can read Rosenberg and drool over the fish and then read the other one to plan your dives.