This Hamburg feature would not have been possible without the following sources, which contain photos and information and are overall excellent books!

Mach Schau! Die Beatles In Hamburg [German] by Thomas Rehwagen/Thorsten Schmidt

Beatles Hamburg Guide [German] by Ulf Krüger

Beatles in Hamburg (Lexikon) [German] by Ulf Krüger

The Beatles in Hamburg [German] by Jürgen Vollmer

Warum spielst du Imagine nicht auf dem weißen Klavier, John? [German] by Klaus Voormann

Die Beatles [German] by S. Nidergesaess

Die Beatles - Ihre Karriere, Ihre Musik, Ihre Erfolge [German] by R. Moers, W. Neumann, H. Rombeck

Das Grosse Beatles-Lexikon [German] by R. Bratfisch

"Internationale Pilzvergiftung" [German] by Bernd Matheja

(For the letter written by Pete to the German Aliens Police): The Beatles The True Beginnings by Roag Best with Pete and Rory Best

Hörzu (German magazine)