One of the best examinations of Luther the man was written by Herbert David Rix. It’s incredibly hard to find at a reasonable price these days: https://books.google.com/books/about/Martin_Luther.html?id=FQB_TKWpHnwC
Interesting; here’s a review of it:
http://art-of-attack.blogspot.com/2012/06/best-book-on-martin-luther-now.html?m=1