My teacher recently told us that he has written multiple(?) mystery novels, but he won't tell us what it's called, and they're published under a pseudonym.
This is all I know about the book:
It was most likely written in German. (A student in his previous class presented it as a book report for German class, but wasn't aware his teacher wrote the book)
The main character is named Johann Friedrich von Solland (or something similar-sounding, because I don't know if it's actually written that way), but it might just be a name the character takes on temporarily, because I couldn't get any results from Google under that name.
Johann is a detective who used to work for the German intelligence agency
Johann has blond hair, blue eyes and is tall. He's also a stereotypical German (hardworking, nitpicky, etc.)
He lives/works in Great Britain and most of his cases are in manors
The plot takes place in the 1920-1930 (My teacher said the 20s - 30s, so it could also be 1820s to 1830s, but it seems unlikely)
Johann has two assistants, one of which is a lady and the other is a man (I think?). One of them is similar to Dr. Watson's role in Sherlock Holmes, as they are the one explaining what Johann does.
The last book that was published came out about six years ago.
I don't have much more information other than that. I'm just really curious to read his book, because I personally enjoy mystery novels, but my teacher won't divulge anything else.