Complex sentence: One "really important" source for him was a series of Polish pamphlets published after the war detailing what happened to the Jews by region, from which he was able to get very detailed information about Sosnowiec.
Split into simpler sentences: One "really important" source for him was a series of Polish pamphlets published after the war which detailed what happened to the Jews by region. From this he was able to get very detailed information about Sosnowiec.