Erich Sander (1903-1944), son of the famous photographer August Sander, was imprisoned in the Siegburg penitentiary from September 1935 until his death in March 1944 for resisting the Nazi regime. He maintained extensive correspondence with his parents. Since 1941, he was able to smuggle letters out of the institution past the censors, some of which were written in invisible ink. The edition includes 144 letters from Erich Sander as well as his parents Anna and August from the years 1935 to 1944. It is a unique documentation of prison life during National Socialism, providing insights into a largely unknown phase of the life of August Sander's family.