The authoritative source for information on perhaps the most obscure of the infamous extermination camps run by the NSDAP regime in its occupied territories 1939-1945.
This is a clumsily translated but fascinating hodgepodge of information about the Chelmno Extermination Camp. A lot of the things included are, as the introduction notes, "practically unavailable for the average reader who does not study history professionally." Included are the results of an archaeological dig on the site, testimonies from the only three survivors of the camp, testimonies of civilians living in the village nearby, and more.
Between 150,000 and 300,000 people died at Chelmno, but hardly anyone has ever heard of the place, probably because there were so few survivors. This book, though short, packs a lot of info in its pages, and I think it's essential reading if you want to know about Chelmno. The English translation is so hard to find, however, that I think a person would be unlikely to read the book unless they had an intense and/or professional interest in the Holocaust.