I wouldn't be surprised to see that show up next book as part of the solution to dust and routes, but I don't really think it's that critical. But I do want to see where they take Jethri. At one point we thought that there would only ever be one Jethri story (and it was a very good one), but I'm quite glad they were able to tell us more about him.
As for the mainline story?
Summer, 2023, according to Ms. Lee. With a fourth Jethri book Fall of 2023 (same blog post.)