A few weeks ago, a non-AH friend I happened to subject to "Making up fictional naval and military orders of battle is my new mini-hobby" rambling suggested the works of Avalanche Press. I got a handful based on either Plan Z or a "WW2" centered around a surviving German Empire, and finished the first one today. The Long War - Plan Z is an expansion for Avalanche's "The Long War" AH setting that - well, as it says on the tin, it's about Plan Z. It's a fairly short book (47 pages, and most of them focused on rules and battle scenarios), but I enjoyed it. First, it's upfront about how much of a grandiose fantasy Plan Z was in terms of German's industrial and financial limits, and the imbalance between the German and British fleets even in a scenario where the author tips the scales in Berlin's favor, but big navies go BOOM! so just roll with it. Second, it's refreshingly anti-Wehraboo - for instance, it frequently points out the often-significant design flaws in Nazi warships; likewise it avoids the lead paint-guzzling you often see in these scenarios and has a larger Royal Navy built up in response. The AH 'meat' is only 9 pages, but it's a good entry level book for this kind of thing.
Next up is Ships of Plan Z, which goes into more detail on the ships that would make up the Uber-Kriegsmarine.