Even if you don't understand any German, this is a treasure. The entire archives of Art Nouveau magazine published between 1890s to the 1930s are now online in this massive archive. There are sketches, covers where the typography is as rounded as clouds, and dark girl in the woods images. The last example is a warning, you could get lost in these archives. Bookmark and enjoy.
Jugend-fans rejoice, the entire archive of Art Nouveau magazine are now online!