Northern & Central Midwest
This Diagnosing Tree Problems Web site from Iowa State University Extension can help you figure out exactly what you need to know to begin diagnosis of a tree problem. The site walks you through each step of the process, starting out with exactly what you need to look for. Then it gives you excellent color photos of the different problems. There are also references for other sources on the Iowa State site although they are not active links.
The Meaning of Trees
After you've identified and fixed any tree problems, open The Meaning of Trees: Botany, History, Healing, Lore, by Fred Hageneder (Chronicle Books, 2005; $19.95), for a lovely trip through the world of trees. It has magnificent photographs, and although not all the trees featured are Midwestern, it is certainly an interesting read.