Long-time contributor Jerry Miller has authored a book: American Occupation of the Rhineland 1918-1923. The book has just been published by the Society on Amazon. See link below from Amazon for book ordering.

On December 1, 1918, having advanced slowly to allow the German army to retreat before them, Allied forces including nearly a quarter of a million American troops of the newly-formed Third Army, crossed the German border. Each of the major European armies had its own zone of control West of the Rhine, with a bridgehead extending 30 kilometers East of the principal crossing in each of the respective zones. The Americans continued to occupy their zone in the Rhineland through late January, 1923.

This monograph explores the history of the US Occupation of the Rhineland through postal material — envelopes and postcards sent by American military and civilian personnel in the occupied area — plus a small selection of non-postal ephemera. It will be especially useful to postal historians but also to people interested in this phase of German-American relations.