The Sharing Knife! I loved those books. I found I was happy to accept the age difference romance as one of those things that I have totally different opinions about IRL and in fiction, and the worldbuilding was full of interest for me - as you say, the material culture, the technology changes, it really tapped into the same vein of satisfyingness as playing house did when I was six. And I also loved all the Tolkien echoes, how it was so much an anti-LotR and at the same time a really loving homage of it.

Five Red Herrings is very weird about Scotland. Though what I find intensely aggravating about that book is the 'here is a clue but I'm not telling you what it is' thing, but I am not the sort of person who reads murder mysteries like I'm trying to solve a crossword.

And none of those are on my list of top favourite Heyers - completely agree that April Lady is kind of forgettable - though I would rate The Nonesuch higher than you did because I like Ancilla as a character and I like the way the setting's done. And Hester and Sir Gareth are a delight!