https://www.geschichtlicher-buechert...nenfabrik.html
Holger Schlemeier completed this history of Erma models and production a bit ago. Unfortunately for me, meine deutsch is nicht gut genug to properly enjoy it easily, and I'm waiting for the English version, which has no projected publication date of which I'm yet aware--although I believe one is intended who-knows-when.
There are others here who own or appreciate Erma's offerings, if only the various toggle action Luger Wannabes, or their .22 lr conversion kits, so this book might be a nice addition to the libraries of those who have the interest.
Its 159,00 € price tag seems not too bad a deal for what I think will prove to be a conscientious, comprehensive, and very informative work. (And somewhere among its pages may be a pic of the markings on an Ithaca Model 72 saddle gun in .22 lr taken and submitted by yours truly at the author's request!)
Way to go, Holger!!