The Reformation, by Diarmaid MacCulloch

I have to highly recommend this book. Not only is it authoritative, highly detailed, and a good read, it's also a great reference book. The chapter on Luther is very good, although not exhaustive.

Moments of revelation in this book include how the black plague had a lot to do with the innovation of the "sacrifice of the mass", and the number of churches all around Europe with many alters where the Eucharist was performed several times a day as many times as possible to appease God. Also, the author gives us quite a vivid picture of the politics of the times, including Jacques Lefèvre d'Etaples, who published commentaries on Paul's epistles in 1512, using language that described human works as "irrelevant" in God's salvation of humanity, as well as profiles for Cardinal Gesparo Contarini and others who joined Luther's reformation.

More importantly, the book traces the aftershocks of the reformation all the way to the 1700s. This book takes a bit of time to read (708 pages), but it's worth every page.