Some people believe on doing the marketing research ahead of time to find out if there is a niche for their type of work. I have never done that, but I would not discard the wisdom of doing so.
I only write about what I am interested in and feel passionate about it.
Once the work is finished, then I start my research to see where it fits best. All my writings are non-fiction so that makes it easier to eliminate all the rest. For the book “Disposable people” that I am working on now the most appropriate category would be “social issues” at the cross roads with “political regimes and history”, intersecting “memoirs”.
After delimiting the category, one needs to find out which publishers are interested in such subjects and that can be done just by going to the library or book stores and looking them up. From that point on all the necessary marketing techniques to get the book published can be found in my hub “How did I managed to get published a few times over”