There's been tons of serious work, but unless I miss my guess, SS-18 doesn't read German.It's very disappointing that there hasn't been serious work on that. The German constitution was one of the most complex and difficult to craft in the modern era, and the fact that it worked so good was rather amazing. We've got a jillion odd books about the American constitution, almost none about the German.
Also, there are problems with a lot of the serious work that is in English because it's horribly tainted by Sonderweg historiography and I wouldn't want to use it as introductory for anyone. Pflanze's own opinions on Bismarck aren't totally congruent with the Sonderweg, even if I don't agree with some of them, so they make for a better introduction.