Today, I find it rather easy to answer that question about Franz Stassen. To start with, Franz Stassen's work was limited mostly to illustrations in books printed in Germany in the early 1900s (he died just a few years after the end of WWII). Now, let's dig a little deeper into why plenty of people would be happy if you never knew Stassen's name. Stassen was close friends of Siegfried Wagner. Not only was Siegfried the son of anti-Semitic posterboy Richard Wagner, but Siegfried himself was honorary president of the Nationalist Bayreuth Federation of German Youth (Bayreuth is a town in Bavaria where Siegfried was also conductor of its annual music festival). Now, if I told you that Stassen began his artistic career as a Naturalist (where the visual truth takes precedence over interpretation), it will probably come as no surprise that Stassen created four tapestries for Hitler's Reich Chancellery--showing scenes from the Edda sagas no less. (Sure, you remember the Edda; I talked about it briefly when I showcased Lorenz Frølich's work back in January of 2012.)
Oh. Wait! Did I mention that even though Stassen was married and his wife died about 30 years before him, he lived his life in the closet? (BTW, so did Siegfried.)
And then there's the part about how much of Stassen's work was destroyed during the war. Nonetheless, during his career, Stassen illustrated about 100 books (many of which feature page after page of his illustrations), 50 bookplates, and 25 postcards. Stassen's work became influenced by Art Nouveau, and he seemed to bounce easily between the more realistic and the more stylized in his illustration work. There are examples of both below. Compared to his more-detailed work with less variation in line weight, his illustrations with less cross-hatching and strong bold lines around the figure edges feature the visual trademark of the Art Nouveau illustration style made famous by Eugene Grasset.
All of the illustrations below come from the book "Siegfried Wagner and his Art," published by Breitkopf & Härtel in 1911. The first image accompanies the entry for Der Kobold (The Goblin), a story replete with a nymph, some fauns, and infanticide! (Turns out, this is something with which Siegfried seemed to have a preoccupation. There's an illustration in the book of a nude male child with two knives plunged into his chest and stomach--sorry, folks, not gonna show that here.)