Reddit reviews The Waning of the Middle Ages

The codes of conduct were developed at the end of middle ages, as the knights were becoming irrelevant in their traditional role, probably as a way to socially consolidate and differentiate themselves. Even than, it was more of ideal than real guidelines.

I'm not a medievalist, so you might still get a better answer. But, for your question Johan Huizinga, The Waning of the Middle Ages is a must.