There is yet another book coming out on woody ornamentals. Mary Kay Malinoski and David Clement at the University of Maryland have written a diagnostic field guide on broad leaved woody ornamentals coming out in March. It is based in part on the Maryland plant diagnostic web site and has color keys to symptoms and causes of problems. Here is a link to get more information.
It was primarily geared toward the northeast region, but may work for other regions. They worked really hard to make this thing practical and useful for just about everyone. Eventually, they plan to put out companion field guides on needled evergreens and herbaceous ornamentals. If you have any questions, feel free to contact David Clement.