Great question. I was thinking that myself recently. Is there a market for publishing books writers would specifically use for research in writing? Or is there a market for writing short encyclopedias or reference books about a very specific area.
There are several "101" books out there for sale on Amazon on various subjects including astronomy (below link).
https://www.amazon.com/Astronomy-101-Wormholes-Theories-Discoveries-ebook/dp/B00DV1V7LQ