Castiglione's Class
Only for serious scholars of the Italian Rennaisance since Castiglione is mentioned in many contemporary poems ,government papers and in everyone's letters at the time .He also mentions many inside anecdotes about famous nobility in this work.
The work however is very dry for the average reader as it relates to manners and characteristic of courtiers and the ladies of the court. Modern readers might find that the nobility had too much time on their hands ,their "games" of discussing the mannerisms of the nobility not much fun, and concerns somewhat silly.(or maybe that's just me!).
It does seem to define the Western concept of "class" not in the political sense but in the social sense("A class act")