The descriptions of underground resistance methods are frank and unglamorous. I found that fascinating. It also depicts the horror of being ruled by the Nazis, with their arrogance, cruelty, and mechanized oppression, such that "ground under the jackboot" makes perfect sense.
Karski also witnessed the horrors of the Warsaw Jewish ghetto and, disguised as a guard, saw mass murder in a death camp. He was one of the first to raise the worldwide alarm on Nazi genocide against the Jews, and had private audiences with FDR and countless others. His testimony is powerful.
I'd be interested to know if my friends in Poland have read this. I would expect it to be required in schools there. Poles generally seem to have much better knowledge of their history than we Americans do of ours. Living in the borderlands between Germany & Russia is one way to keep history relevant, unfortunately.