We have probably out most specialized book yet for you today, entitled History Writ in Steel: German Police Markings 1900-1936. As the title suggests, it is about firearms (and to a lesser extent bayonets. rifles, and holsters) with German police property markings. The structure of German police units was pretty complex during this time, and there are dozens and dozens of heavily abbreviated markings used by them.Donald Maus has done a very good job with this book of explaining what the markings mean, and providing a resource to allow you to identify where a particular piece came from.
History Writ in Steel is available directly from Simpson Ltd.