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This is a long read (400+ pages), but you end up with a very good understanding of Luther's theology. Althaus cites Luther so much, both in the text and in footnotes, that it reads almost as a primary source. Imagine giving someone the task of reading all of Luther's Works and then distilling that into 450 pages. Althaus is more than up to the task.
I found the chapter on the community of saints to be particularly interesting along with being needed at the present time. The church catholic should focus on the community of saints and on the corporate nature of worship and faith. Luther's words on this prove helpful.
The common thread throughout this work is, not surprisingly, justification through faith. There are some very interesting tangents and corrolaries that at first don't seem like they have much to do with sola fide, but in the end are directly related. Luther's christology is one example.
Unfortunately, I discovered that Althaus was a supporter of Hitler. That opens up a whole 'nother can of worms. I'm not sure what to do with that information, but it certainly needs addressed.