Protect Your Garden by Ed Rosenthal, perhaps the best guide I've come across to identifying and dealing with pests, diseases, nutrient deficiencies, and environmental stresses--in other words, all the nasty things that can happen in your garden.
You may know Rosenthal as the leading expert on the cultivation of marijuana. (Don't worry--I won't judge you.) Having written more than a dozen books on one of the greatest cash crops ever grown, Rosenthal is perhaps used to thinking of garden protection in more interesting terms, but in this book he chose to focus on protection from the kinds of everyday threats that any gardener can come up against: bugs, vermin, viruses, bacteria and fungi.
Protect Your Garden is loaded with full-color photos so it functions first of all as a terrific aid in identifying the problems you may discover. You'll find information on how common the pest or disease is, what kinds of plants it attacks, what kind of damage it does, and how to control or prevent it.
Following the sections on pests and diseases (which comprise about half the book), there are sections on nutrients and environmental stresses so you can diagnose and treat nutritional deficiencies or problems like salt injury, frost damage, or overwatering.
The last section outlines a wide range of eco-friendly and biological controls, filling in more details about how the controls work, how they should be used, and who makes them commercially available.
Protect Your Garden is the kind of book you never want to need, but you will. Seriously, you will. There are only a few books that I would say this about: It should be on every gardener's shelf.