As the title suggests, I want to start a thread about titles, the rules of precendence, etc., because despite having read several period etiquette books and a lot of guides, I still frequently find myself going 'wait, does the wife of the younger son of a baron come before the wife of a baronet?' (not an actual question, just an example). Here are the resources I've found helpful so far:

A very useful basic guide by historical romance writer KJ Charles, including a very good argument for why getting this stuff right is important.

Charts indicating the correct title and form of address to be used in various occasions.

Debrett's Guide to the Peerage

An 1893 book Etiquette for Every Day (Chapter 3 has an exhaustive list of the order of precedence).

To start things off, my (current) question: how do you decide what mode of address to use in writing that isn't dialogue? There are different ways to address someone in formal correspondence, informal correspondence, speech, etc. etc. but what about when it's inside your character's head? Is it useful to introduce a character with their full title the first time, or should you go with whatever your character would call them if they were speaking aloud? What do you do with titles like 'Baron', that nobody ever actually called someone in conversation or correspondence? How do you indicate that someone is a baron when nobody would ever call him 'Baron'?