I came to this book having done little study formally of pietism. Outside of the occasional quick, mostly derisive, reference, I would continue on my way. Often, theologians do more to try to talk about how their *not* pietists than actual discuss what pietism is.
Roger Olson, America's favorite belligerent anti-Calvinist warhawk, has given us a delightful little introduction to a movement often maligned, and seldom understood. The book is a handy tool to understand Pietists major figures, historiographical issues, and why more people ought to recognize the palpable influence on American evangelicalism. I think that Olson's central thesis is basically correct: unless you understand the pietistic roots in the lives of men like Whitefield and the Wesley's, you will have trouble understanding concepts like the New Birth, or importance of affection.
Though feeling a little cynical at times, this is a nice, short introduction to Pietism. For anyone curious about the history of evangelicalism, I would heartily recommend.