Grahame,
I'll second tom's recommendation of the Bird Finding Guide and also suggest 'A Guide to the Birds of Mexico and Northern Central America' (Steve Howell & Sophie Webb) (only shortcoming of this book is that it lacks shore birds so you need something else if you want them, otherwise it is heads above the rest).
For some of my trip reports on birding in Mexico, use the search feature on Birdforum and search for posts started by Dave B Smith and the word Mexico in the title and you'll find about 10 reports that may be useful.
If you are specifically interested in the Quetzal, I'd follow curunir to Costa Rica and head to Monte Verde. In Mexico, El Triunfo is probably the best site for this bird, but that is a hike and camping, and probably a guide required.
Good luck and please let us know what you find. It is great birding!