This is the best book that can be found on this subject, which is not the same as this is a very good book, but I doubt that there will ever be a much better book on this aircraft. When it appeared in Germany as "Bv 138, der Fliegende Holzschüh", the German publisher Podzun Verlag had tried to get all remaining photographic material. They didn't use all available drawings though, you can find some additional ones in books as "Warplanes of the Third Reich" or "Warplanes of the Second World War, Volume five, Flying Boats", both by William Green or in "German Aircraft of the Second World War", by J.R. Smith and Antony Kay.
But you won't get much more information from those books on this plane, and hardly any extra good photograph.
On the whole I would say that with a bit more unit history and a good colour art section this book might earn its fifth star.