There are many books not mentioned in this post that contain valuable information.
What I have found is that for the collector and researcher, one needs to go back to the beginning and move forward, understanding that the early books were in many cases " best efforts" and filled with errors by today's standards. Yet the old books do contain information that has not been duplicated in modern productions.
It is sometimes beneficial to return to those old books to fill in the holes left by modern productions.
I like " German Small Arms Marking" from authentic sources by Gortz & Bryans 1997, seems like yesterday but not at all.
One of my favorites
Vern