I Don't ended sort of abruptly. Martin Luther hit the scene, shook things up, and then -- splat. The End. But I see that the author has a sequel planned, and I'll look forward to the next 400-or so years that when the time comes.
Luther -- he touted love and "constancy" as big plusses of marriage. Actually had some semi-progressive ideas (for the time, anyway) about relations (sexual and otherwise) between men and women. Told couples that embers -- the still-glowing remnants of their passion -- were to be reveled in as much as their fiery predecessor. His ideas about marriage were as radical as the ones he had about the church. I came away from those chapters wanted to drop into a Lutheran service. At some point, I will. Did you know he married a nun he'd busted out of a convent? And that he was a bit of a hypochondriac?
One quibble (or question): how could a book about marriage in this era fail to mention Henry VIII, Catherine of Aragon, and Anne Boleyn?