Hi Stewart
There are a couple of site guides to Costa Rica that are very useful. I have included below the brief summaries that I have on my website. I bought both books when I was in Costa Rica, so I don't know how hard they will be to find outside of the country. Denis Rogers runs Cinclus Tours, which I think has a website. Aaron Sekarak runs a small hotel not far from La Selva.
I hope this is of some use. However you organise it, great birding is guaranteed.
Tom
Rogers, D. W. (1996) Site guides: Costa Rica and Panama. Cinclus Publications.
This is a very useful guide to Costa Rica. It includes a number of sites that are not included in Sekarak and I have used both books regularly when planning my Costa Rican trips. I have still never visited Panama, but I guess that the book will be equally useful there. It has a useful summary table covering 27 pages, which gives information on the status of each species in various regions of the two countries. This and the index, which Sekarak does not have, help to locate the sites to see any species that you are particularly after.
Sekerak, A. D. (1996) A travel and site guide to birds of Costa Rica. Lone Pine Publishing.
This guide covers most of the major sites in Costa Rica. I’ve spent about 11 weeks traveling in the country and I’ve found this book to be very useful throughout and it is the main guide that I have used to plan my trips. The maps are particularly good. It includes a useful set of tables that give information on each site. Another very useful section is a table, extending over 5 pages, that gives information on most of the Costa Rican Specialties. This is helpful to plan out the important target birds, especially if you are not very familiar with the species in the country.
B 
