This synthesis of the growing body of information from research on epiphytes and their relations with other tropical biota provides a comprehensive overview of basic functions, life history, evolution, and the place of epiphytes in complex tropical communities. Epiphytes comprise more than one-third of the tropical vascular flora in some tropical forests. Growing within tropical forest canopies, epiphytes are subject to severe environmental constraints, and their diverse adaptations make them a rich resource for studies of water balance, nutrition, reproduction and evolution.
My edition says published in 1990, so I wonder about the places where it notes things are conjecture and/or poorly studied, but it's a very nice overview on epiphytes.