DKW motorcycle books includes the history of DKW (German text) plus a magazine article.
DKW is an historic car and motorcycle marque founded in 1916. A two-stroke toy engine was built in 1919 and a slightly modified version of this engine was put into a motorcycle and called Das Kleine Wunder - 'the little marvel'. This was the real beginning of the DKW brand: by the 1930s, DKW was the world's largest motorcycle manufacturer.
DKW Motorrader 1949-58 by Stefan Knittel (1993). German text.
The Classic MotorCycle (January 2000) includes an article on 1935 DKW (Das Kleine Wunder) SB500.