The Rise of Christianity: How the Obscure, Marginal Jesus Movement Became the Dominant Religious Force in the Western World in a Few Centuries, by Rodney Stark. Joining sociology and history, Stark contributes much new insight to the amazing rise of Christianity in the Roman Empire—and he reaches some surprising conclusions.
The Barbarian Conversion: From Paganism to Christianity, by Richard Fletcher. This is an astonishingly thorough study of how most of Europe was reached by the Christian faith, often badly and more often superficially, from the fourth to the 14th centuries. It's stimulating reading for anyone interested in rethinking evangelism.
A Secular Age, by Charles Taylor. Effective evangelism in Europe and North America now occurs in a post-Christendom context and therefore requires an understanding of that context. A number of very good books have delineated the decline of Christendom and the rise of secularity and postmodernity. This is the best of the lot.
Alcoholics Anonymous, fourth edition. The recovery movement is the underground awakening of this generation. More people experience initial liberating grace in that movement than in churches. AA's classic "Big Book" is essential—and inspiring—reading for Christian leaders.
Understanding Religious Conversion, by Lewis R. Rambo. Few Christian leaders have a nuanced understanding of how people become Christians. Rambo draws from an astonishing range of sources to demonstrate that conversion is a process that takes place over time, in approximate stages, with several things going on simultaneously. Someday evangelism will be informed by "applied conversion studies."