I used the sixth edition of this book for the basis of a Maryland Tree Expert certification exam course I developed in the late 90's.  I found it to be an excellent introductory text on a number of issues related to tree care; tree selection, planting, soils and pruning.  There is also a very good overview of the mirad of tree insects, diseases and other problems, --a topic which might otherwise seem overwhelming in its scope.  If you are new to the practice of tree care or looking to prepare for certification, then Pirone's Tree Maintenance is a great place to start.