Very good overview of Martin Luther's theology. I really liked the part on the community of saints and Martin's support for the baptism of children. The book gives you a good base from which to read Martin Luther's own works. I am currently reading his comments on Galatians. I would highly recommend it.
This is an in-depth, well-referenced overview of Martin Luther's theological thinking. It is a classic. If you want to know Luther's theology, this is the place to start. If you want to be entertained, read Luther's Table Talks.