If you're looking for a theological thriller, this is not the book for you (even though there is some great theology contained therein at some points). However, if you'd like a peek into the fishbowl of the legendary Luther family, then pick up this short and moving read.
An interesting tidbit about Kate (spoiler alert) is that she was essentially disowned by her father for leaving the convent (and, of course, marrying Martin).
This biography teases out the quote from Luther on p. 66– "The first love is a kind of intoxication; then when this [kind of love] had worn off then comes the real marriage love."
As high profile as they were, the Luther marriage was very normal and gritty. The book captures well the highs and lows, the joys and sorrows of family life.