Having fun with low-budget covers

The theater publisher Albert Langen / Georg Müller in Berlin used a cheap but charming cover for low-budget scripts, like this play (Onkel Theodor by Selma Lagerlöf). 

By simply opening the cover, readers noticed that it had a window in the style of a stage. The effect was a little like the curtain rising in the theater. These covers were applicable to any play, since the title of the play was printed on the title page which, originally, seemed to be part of the cover. 
Other publishers used this technique in more prosaic ways, e.g. for the cost-efficient publication of dissertations.