If you would like to read a well written, IMO, self published book, get a copy of J P Clabrough by Larry Shelton. I understand it took him a long long time to research and write.
Very interesting read for any English shotgun enthusiast.
Everyone has a different style of research. Some peruse old magazines, others turn to the census records. Larry's approach is very unique. He ferrets out living descendants of his research target. He then sits with them and listens.
I have nothing but high regard for him and his results.