"Modern Arboriculture": I just purchased this book at a library book sell off for 25 cents. Available at Amazon for $ 102.00 new. Score!
This book is like a university level course in tree anatomy, physiology, and arboriculture practices mostly in illustration and caption format . The simplicity of the format is deceiving. Each page requires some study, go outside and look at a tree research . A page or two a day is all I can absorb.
What a fantastic find. I highly recommend it.
Addendum: The knowledge in this book was gained by the author himself when conducting autopsy / dissections of 15,000 trees . Mr Shigo is the Galen of the tree world!
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Dr. Shigo did so much to further our knowledge of how trees live and respond to human interaction. He is considered by many to be the father of modern arboriculture.
As mentioned by Wayne, this is a very high-level discourse on how trees function so it can be a little much for the lay-person. This book isn't really for someone with just a passing interest in trees but is an amazing resource for someone who want's to know every single detail about how a tree works.
"Instead of Pay It Forward I prefer Plant It Forward" ~Howard Story / "God has cared for these trees, saved them from drought, disease, avalanches, and a thousand tempests and floods. But he cannot save them from fools." ~John Muir