Anatomy of Trees by Rex Vicat Cole
The Artistic Anatomy of Trees 
First off, I am a fan of anything written by Rex Vicat Cole. He always covers topics in such an organized and thoughtful way. Making it easy to read through and begin to apply his lessons to your work immediately.
All the diagrams are in black and white, however this does not matter, because what Cole is teaching is about the basic principles of landscape paintings and how trees fit in.
There are also a few of his books available for free via Google Books, here are the links I could find:
British trees drawn and described By Rex Vicat Cole
British Trees, Volume 1 By Rex Vicat Cole and Dorothy Kempe
