Complex sentence: At that time he was a self-employed translator for the editor Stuttgarter Motor-Buch Verlag and, under the pen name Rainer Buschmann, he translated books devoted to military history from English to German.
Split into simpler sentences: At that time he was a self-employed translator for the publisher "Stuttgarter Motor-Buch Verlag". Under the pen name of "Rainer Buschmann", he translated books devoted to military history from English to German.