At the Water's Edge: Macroevolution and the Transformation of Life
New York: Free Press, 1998. 304 pages.
During one important period of life´s history, vertebrate creatures left the water to colonize land, and later, some vertebrates readapted to that environment. Zimmer traces the discovery of both the transition to land of early tetrapods, and the later transition to water of the whales. If you are ever having an argument over "transitional fossils", this is the book you want to have!