Best guides available for the following areas IMO:
Ecuador: The Birds of Ecuador (Vol. II -- Field Guide) by Ridgely & Greenfield (2001). Too large to use in the field (some bind the plates separately), but high quality text, plates, and maps (though maps are separate from plates). Volume I has more extended discussion of Status, Distribution, and Taxonomy, plus introductory materials providing an overview.
(More detailed information on books also recommended above):
Costa Rica: The Birds of Costa Rica by Garrigues and Dean (2007). Nice, compact modern-style field guide, with quality text, plates, and maps altogether. A Guide to the Birds of Costa Rica by Stiles and Skutch (1989) is a large, old-style guide but still useful as a reference because it has more extended textual discussion and info than the G&D book.
Mexico and N. Central America (Guatemala, Belize, El Salvador, plus): A Guide to the Birds of Mexico and Northern Central America by Howell & Webb (1995). Very high quality and extended textual discussion of ID issues, accurate plates, and maps. But too large for the field, and old-style with maps separate from plates. Some bind the plates separately. Covers countries mentioned plus parts of Honduras and Nicaragua.
Jim
[Edited to make clearer I'm agreeing these are the best available guides even though I mention drawbacks.]