I do not collect French or German, but had a few pieces of family correspondence from Interwar Alsace that I thought I might someday look at more closely. Never got around to it, but I did become aware of some titles that might be of interest. I have not seen them myself, though.
Catalogue des timbres à date français du Haut - Rhin 1915 1940, JP Bournique, SPAL 2000 -
Catalogue des timbres à date français du Haut-Rhin 1944 - 1947 - avec millésime à 2 chiffres - Alsace Lorraine - SPAL
The SPAL is a group that is devoted to postal history and cancels of Alsace.
Not sure where you might get these in the US, but Delcampe has them for sale
See
https://www.delcampe.net/fr/timbres...ace-lorraine.
Haut-Rhin is only half the region. Bas-Rhin I did not find equivalent publications, so I stopped (my correspondence originated there) .
- Jonathan