"Photosynthesis" by D. Hall and K. Rao. This is the fifth edition (1994) of a classic reference first published in 1972. If you want to understand trees, you MUST understand photosynthesis. This is a really good place to start. It is published by Cambridge University Press (ISBN 0-521-43622-2) in paper for a reasonable price. With its 211 pages, it is long enough to contain all the detail we mortals can handle, but yet detailed enough for those that really want to get into the subject. A really good book.