Galateo: Or, The Rules of Polite Behaviour by the late medieval Italian author and diplomat Giovanni Della Casa is his vintage etiquette manual, free for a limited time courtesy of the University of Chicago Press.
This is their featured Free Book of the Month selection for December. The manual was originally published in 1558 as Il Galateo, overo de’ costumi and was highly influential back in the day, widely circulated for centuries after its initial publication in assorted translations, and can be considered a precursor to modern etiquette authorities such as Miss Manners.
This edition is newly translated by M. F. Rusnak, and contains a facsimile of the frontispiece of the older 1576 English translation, alongside all sorts of wittily-presented practical advice for ordinary Renaissance persons aspiring to gentlemanly refinement on conversation, fashion, table manners, as much for How Not To Do It as well as how to.
Offered worldwide through December (will probably stay free a day or two longer due to the New Year’s holiday), available directly via the university’s website.