Hm, just finished _The Black Thread_. I really wish I’d read the two novellas in the opposite order.
I think _The Red Threads of Fortune_ works well either as a continuation or a standalone novella. However, from a plot perspective, I didn’t feel as much resolution with _The Black Tides of Heaven_. Probably, because I already knew about Eien.
That said, there were a lot of interesting topics that were explored more deeply in this book and cleared up some questions I had.