On 19 September 1940, Witold Pilecki, a Polish soldier, went outside during a street round-up in Warsaw to be captured. Along with around 2,000 civilians, Pilecki was taken to Auschwitz, where he was assigned the inmate number 4859. He had volunteered for a secret undercover mission: to get inside the camp, gather intelligence and report back to the Allies.

Under the pseudonym Tomasz Serafinski, he arrived at the camp between 21 and 22 September and saw Auschwitz for the first time.

"We were struck over the head not only by SS rifle butts, but by something far greater," he wrote. "Our concepts of law and order and of what was normal, all those ideas to which we had become accustomed on this Earth, were given a brutal kicking."

Pilecki's clandestine intelligence, received by the Allies in 1941, was among the earliest describingthe appalling conditions of the Nazi death camp. His work, published into English for the first time in 2012, stands among the few diaries of camp inmates, alongside the works of literature and memory by Primo Levi and Elie Wiesel...