Peterson pioneered field guides for American wildlife identification and they are still the best choice for many groups in North America. However, I think Peterson stretched himself in trying to cover the massive Mexican avifauna. And this particular Peterson has not kept pace with the much improved, newer field guides, so it has now become all but obsolete. It's still useful as an aide memoire for the pocket, but for the last ten years the best field guide to Mexican birds has been Howell and Webb and it looks unlikely to be superceded while I'm still birding. Though Edwards is an interesting book, it is hard to use it as a real field guide. So, either buy Howell and Webb or wait for Peterson to be thoroughly revised or perhaps for another pocket-sized guide to appear.