Hello:
I almost totally missed this one. First, welcome to Birdforum! (Despite appearances to the contrary) you will find a wealth of helpful information here. And people. Usually. [*ahem*]
I, too, have been researching Puerto Rico. As far as I can tell, the book
Birds of the West Indies by H. Rafaele
et al. (Princeton Field Guides, 2003) is the best choice. Some discussion already exists on BF:
http://www.birdforum.net/showthread.php?t=30188&highlight=west+indies
As you can see, it is already a little out-of-date, but I do not know of any truly newer book (some reprints, but not a genuinely new book).
There is also a stand-alone
Guide to Puerto Rico and the Virgin Islands, by the same primary author, which I know nothing about. However, if you plan to go back to any other Caribbean island ever, I suspect that the more comprehensive book is probably the best value.
One thing I don't know about either, is how well they cover wintering N.A. birds - I know my old Peterson Guide (
the James Bond) didn't illustrate these at all. You may want to take a regular N.A. guide as well.
Peter