A Natural History of Costa Rica by Daniel H. Janzen (1983) — Janzen spent decades working in Costa Rican forests and this book, though old, remains essential for understanding what you're looking at. It covers insects, plants, mammals, birds, and the ecology connecting them, with a focus on the tropical dry forest and the cloud forest. The Wilson Gardens and La Amistad become three-dimensional when you've read Janzen on the organisms that live there.
Plants of the Wilson Botanical Garden — the Organization for Tropical Studies has produced documentation of the Wilson collection over the years, and whatever current edition or guide is available at the garden's visitor centre is worth buying. It won't make gripping reading on the bus, but walking the garden with it in hand reveals the extraordinary breadth of what Robert and Catherine Wilson assembled over decades. Ask at the Las Cruces Biological Station reception what is currently in print.
The Quetzal and the Macaw: The Story of Costa Rica's National Parks by David Rains Wallace (1992) — Wallace traces how Costa Rica's remarkable national park system was built, largely through the effort of a handful of scientists and conservationists. La Amistad and the southern zone parks feature prominently. It gives San Vito's hinterland — much of which exists as protected wilderness because of the story this book tells — the context it deserves. A traveller who reads it will understand the southern highlands differently.