Not a roleplaying book, although it sounds like it, and an RPG set in Danzig could be pretty cool, actually, now that I think about it, but the Free City Sourcebook is just-launched site collecting primary source material on that very odd interwar phenomenon, the Free City of Danzig.
Created by the various post-war treaties and governed by League of Nations mandate, the basic theory behind the Free City of Danzig (which is now the Polish city of Gdansk, and the site where the first shots of World War Two were fired) was that because neither Germany nor Poland would let the other rule this important port city on the Baltic, neither of them would. Like most compromises this pleased neither side at all and was constantly undercut by both the German and Polish governments (and by the general weakness of the League of Nations), but the basic theory had some sense behind it.
I haven’t looked at the Free City Sourcebook in massive detail, but there’s a good basic timeline and it looks like an increasing number of links to primary sources, some through things like Google News, some on other third party sites and a number hosted right on the Sourcebook’s own site. It’s always nice to find more people who don’t treat the Interwar Period as some oddball interruption to the two World Wars but as a proper, strange and fascinating historical period in it’s own right. I’ll be following the Sourcebook’s progress with great interest!