You won't surprise any gardeners with the news that hanging around with the plants can be good for you. Sue Stuart-Smith makes a more scientific case--she's a psychiatrist--in the book The Well-Gardened Mind: The Restorative Power of Nature.
Gardening gets people away from the stresses and disappointments of the human world, and connects them to living things and processes that existed long before humans did.
The author visits with stories to tell of gardening in settings from the quiet countryside to the concrete courtyards of Riker's Island Jail in New York City.