Keen does an outstanding job of exploring the concept of "chivalry" in the middle ages. To a degree, this book is essential for a well rounded understanding of the era (it is included on Cantor's Canon of 150 books about the middle ages that every student should read). On the other hand, I'm not convinced that this book doesn't overlap, to a significant degree, with volume two of Marc Bloch's "Feudal Society". Nor, for that matter, was I particularly impressed with the writing style. In fact, I would call this book a snoozer, and I'm not using that term lightly.
Keen's basic analysis revolves around three interrelated aspects/facets of chivalry: social, military and ecclesiastical. So, if someone asks you "What's Chivalry?" you can feel pretty save by answering, "it was a value held by segments of the land owning classes in europe during the middle ages. it had different aspects that had various degrees of importance over the time period in question. These aspects include social, military and ecclesiastical."
Anything more then that and you'll actually have to read the book. One fact that suprised me was the lengths to which the church went to to discourage tournaments.