Diseases and Pests of Ornamental Plants, 5th Edition (ISBN 0471072494)

Dated, but valuable resource:
Look up your plant and read through a whole list of common insect and disease problems for that species. It's that simple. The descriptions are concise and most of the major plant species and their problems are covered. A stong word of caution about this book. Pesticides come and go and the lables change. Since this book was published (1979), most of the chemicals recomended in it have gone off label and are no longer available. Check with your local Extension office for current control recomendations. Aside from that, this is still an excellent place to start for pest identification.