I am not a horticulturalist - just a hobby gardener who happens to also be a botanist. But I still get a lot of questions about gardening and where to find good information. Rather than duplicating some of the extensive information that is already available on the web, I thought I'd share the list of books on my shelf whose edges are most tattered and pages most smeared with dirt: clear indicators that they have been consulted either a lot or right in the midst of gardening crises.
Keep in mind that there are hundreds of gardening books; this list represents a very personal, tiny fraction of what's available and useful.
Bradley, Ellis, and Phillips: Rodale's Encyclopedia of Organic Gardening
Organized by key words A-Z, this has been the fastest way to remind myself about essential information on selecting, planting, and caring for common garden crops and flowers. Entries on general gardening information like composting and organic fertilizers have helped me figure out vexing questions such as how to balance nutrients from wood ash, green sand, and fish meal, or how to make my own soil mix for containers.
Ellis and Marshall: The Organic Gardener's Handbook of Natural Insect and Disease Control
My go-to text when trouble is brewing in the garden. The first part of the book alphabetically lists fruit, vegetables, and some flowers, describing the most common problems for each and suggesting solutions. Entries for most trouble-prone crops have highly useful pictures to help you tell which problem you might be dealing with: what exactly does the damage look like, where on the plant is it -- things you need to know to figure out whether to control insects, fungi, or change culture conditions. Part 2 is a picture-guide to identifying crop-associated insects, both useful and harmful, suggesting control practices for the latter. Part 3 focuses on identification of and remedies for plant diseases.
Hessayon: The House Plant Expert
Plant biology professors get asked a lot of questions about students' ailing house plants. Eventually, it occurred to me that, rather than explain that I learned nothing at all about house plants in graduate school, it was much faster and more helpful to hand students this book. Both text and pictures are packed with useful information, rather than decorative (as in many coffee-table books). Even if the student and I are stumped as to what the plant is, there is a key in the back that lets you figure it out - a feature missing in other books I considered.