The Man Who Planted Trees: Lost Groves, Champion Trees, and an Urgent Plan to Save the Planet is the story of David Milarch, a Michigan nurseryman who, after a near-death experience, devoted his life to creating a genetic archive of champion specimens of all species of trees.  From willows to sequoias to the cedars of Lebanon, he took cuttings from the finest trees he could find, and attempted to clone them so that their characteristics would not be lost.  His reasons for undertaking the project were not scientific, but many scientists support the project.  There is a lot we don't know about trees simply because it's so hard to find out, but author Jim Robbins does a very good job of explaining, in layman's terms, what we do know about them: that they are incredibly important to life on earth.  This passage comes from a discussion of urban trees, and lists some of their benefits:

We shouldn't think of trees as only beautifying a city or suburb, but as a strategically planted ecotechnology, part of a living, versatile, valuable environmental infrastructure that cools the urban heat island, cleans and manages water and air, acts as a natural mood elevator that reduces anxiety and depression, improves property values, mitigates noise, provides wildlife habitat, recreation and medicines, and grows fruits, nuts and other nutritious foods.

Robbins weaves science, history, current issues, tree lore, and Milarch's passage from alcoholic to award-winning environmentalist, into a fascinating and easy read.  It is not preachy, but one finishes the book with an understanding and appreciation of how vital trees are to the health of the planet.