Zündapp motorcycle books cover the history of Zündapp, plus there are several Zündapp manuals available.
The first Zündapp motorcycle was the Z22 in 1921, the "Motorrad für Jedermann" (motorcycle for everybody), a simple and reliable design that was produced in large series.
Zündapp KS-750 In Detail - Zündapp Ks-750 with Sidecar In Czech Private Collections by Frantisek Koran (2000) is a photo manual for modellers.
Zündapp 1922-1984 by Siegfried Rauch (2006). Written with Reiner Scharfenberg and Günter Sengfelder. German text.
Zündapp - 60 Jahre Zündapp-Technik by Siegfried Rauch (1977). Written with Heinz Kletzke. German text.
Zündapp KS 601: A Legend - The Green Elephant by Reiner Scharfenberg (2000) is a concise look into the German Zündapp company history within the framework of one if its most successful motorcycle designs: the KS 601.
Zündapp-Motorräder 1947-84 by Walter Zeichner (1988) covers Zündapp motorcycles from 1947 to 1984. German text.
100cc KS100 Scrambler Motorcycle Owners Manual By Zündapp (date unknown)