This is a good beginner book for any gardener (no matter how inexperienced or experienced) to read. The author discusses the science behind plant growth and gardening techniques in an accessible manner that does not require a biology degree. She also explains the workings (or lack thereof) behind some gardening myths. This book explains how plants work - it is NOT a plant identification guide.
A more detailed book that explains how plants work is: Botany for Gardeners by Brian Capon (third edition).