Forsyth, Adrian, and Ken Miyata. A readable, lighthearted insight into tropical ecosystems. Costa Rica Nature Atlas-Guidebook. Editorial Incafo, 1992. A very useful and readable guide to parks, reserves, and other sites of interest.
Text is backed by stunning photos and detailed maps showing roads and gas stations.
Janzen, Daniel, ed. Costa Rican Natural History. University of Chicago Press, 1983. The bible for a scientific insight into individual species of flora and fauna. Weighty, large format.