Today I visited a library, and picked up a book by an author who had better remain anonymous. Anyway, I dipped in, like you do, and found that the main character was unable to describe herself properly. She literally did not know her own name, or to be more accurate, title. This annoyed me, especially as the novel was written in the first person, which makes it feel even dafter.
Anyway, we all have to learn, and for the benefit of my writer friends who are not as pedantic as me I thought I would set out the relevant rules for English titles, as from (roughly) the fifteenth century. In fairness, I admit things were in a state of evolution, especially in the earlier years of the century, and you might find exceptions out there.
First, and as a general point, please be aware that there has been a lot of title inflation over the years. Through most of the middle ages an earl was a very great lord indeed, especially in the earlier centuries, so if you're inventing a character you might be better sticking at knight level. There were no dukes at all until the fourteenth century and the first non-royal dukes appeared in 1398.
Similarly the title 'Prince' originally only applied to the Prince of Wales, and the random use of Prince and Princess as titles for the king's children really only took off under Henry VIII. You may find that dukes and duchesses are sometimes referred to as 'princes' or 'princesses' as were royal children, but this was not their formal title in the middle ages. It was more a reference to their status. To give an example close to home, Lady Constance of York was certainly a princess, but she was never known as Princess Constance of York, and still less as HRH. The HRH thing is, I think, 18th century, certainly post-medieval.
Second, medieval women always kept their highest title. So if a woman married the Duke of Norfolk, she remained the Duchess of Norfolk, even if she subsequently married Lord Bloggs or Mr Smith. She did not start calling herself Lady Bloggs or Mrs. Smith. This may seem odd to us, but in those days she also kept her precedence and the use of her title helped with this!
Thirdly, the use of female Christian names in titles was (and is) very particular. If Jane Smith marries Sir Fred Bloggs (or Lord Bloggs) she becomes Lady Bloggs. If she's widowed she might refer to herself as Jane, Lady Bloggs. But she can only be Lady Jane Bloggs if she is the daughter of an earl, marquis or duke. In this case on marriage she changes from Lady Jane Smith to Lady Jane Bloggs. Indeed, she's Lady Jane Bloggs even if her husband is Mr.Joe Bloggs, and there's a fifteenth century example for this - if you want to know, Lady Anne Paston, wife of Mr William Paston and daughter of the Duke of Somerset.
This may seem very picky, but in the middle ages a married woman retained her father's rank (in terms of precedence) if her husband's rank was lower. So between two knights' wives, Lady Joan Bucket always outranks Lady Bouquet.
However if a woman marries an earl marquis or duke, then irrespective of her birth she is known as the Countess of Whatever (or Lady Whatever) the Marchioness of Whatever or the Duchess of Whatever. Presumably because if she was known as Lady Mary Whatever she might be confused with her own daughter!
Complex isn't it?
Oh, and the daughters of barons and knights were not 'Lady Jane Whatever' - they were 'Mistress Whatever or Mistress Jane Whatever.'
A lot of this stuff can be picked up by reading Debrett's as long as you remember to scrape off the Tudor and subsequent accretions. Either way, dear authors, please do make the effort as failure may lead to your book being thrown at the wall, and to at least one of your readers tearing his hair out.
Oh, and another thing, if you want to produce a facsimile of a medieval letter, please do try to discover what a medieval letter looked like. The Paston Letters are a good place to start and don't need much research to find. You can modernise the spelling all you like, I'm all for translation, but please try to keep the feel, the flavour of the era you are supposedly writing about, unless you are actually intending to write a parody. Yes I like parody too, but only if the author began with one in mind.
RANT OVER!!! You may now proceed with your lives.