1. I was thinking it would be something from C18th and I‘d have to research publication dates, but then I remembered The Tale of Genji from C11th
2. The number of books in a series isn‘t a consideration for me
3. The one I‘m most sad about is Reginald Hill‘s Dalziel and Pascoe series because I think he had a lot more to say about the state of the world.
Thanks for the tag @Andrew65