I use the Moon's MExico City book. Small, and I have found it reliable. I like the map of the Centro area in that book and have had it laminated to put in my purse. I also like the Streetwise Mexico City map, laminated and fan-folds up easily, and it has just enough detail btu not overwhleming to the point that you can't find things on it.

http://www.streetwisemaps.com/world-map/world-map.html