The American Horticultural Society "A - Z Encyclopedia of Garden Plants", by H. Marc Cathey, Christopher Brickell
We use this as one of our main reference books.
Starting with great instruction on how to use the book, and some basic botany for the gardener, it also has descriptions of leaves, flowers, and cultivation in general. There is some basic pruning and propagation information. Even though over 1,000 pages emphasize identification, there's enough information to get you started in gardening.
Our major use for this book, though, is information on specific plants. It has a great index, with both Latin and common names, helping you find the right plant, fast. For each genus the book describes the different species and gives directions on outdoor and indoor cultivation, propagation, and pests.
For each plant you can find what dimensions to expect, temperature ranges tolerated, and helpful descriptions and photographs so you can identify it.
Truly a book to consider in your reference library.