This is without question one of the best historical works I have read. Perhaps it is the best. Mattingly writes clearly and plainly, even entertainingly, and without presuming too much background knowledge on the reader's part. In an effort to make the book more accessible to general audiences, the author chose to publish not through a university press, as would have been usual, but through a commercial publisher, and his goal of accessibility shines through. Nonetheless it is a scholarly work, and there is a significant collection of end notes for readers interested in digging deeper, with a note at the end listing other notable works on the subject.
Many detailed descriptions are included of events and specific persons in the history of European diplomacy in the Renaissance period, but what I found most compelling was the holistic view of how the role of the resident ambassador and the institutions surrounding it grew out of the medieval Roman law and the ecclesiastical canon law, how these institutions spread from the microcosm of the Italian peninsula to the whole of Europe, and how their foundations in the concept of a united Christendom eventually crumbled to give way to a new concept of "international law" among sovereign nations.
In the process of discussing specific developments in how diplomacy was carried out, the book often steps back and reveals a great deal about broader changes in government, warfare, and accepted worldviews during the Renaissance. This book is a definite candidate for a reread some time in the future!