I've read several Soviet Gulag testimonies, and the value of this one is that Fr. Ciszek, an immigrant American, volunteered to put himself in harm's way in order to serve God and his fellow man. He prepared to take Eastern rite (Orthodox-like) Catholicism behind the Iron Curtain and assumed it as a personal mission. He was captured by the Russian army near the Polish border during World War II and convicted of being a “Vatican spy.”

Jesuit Father Walter J. Ciszek then spent some 23 agonizing years in Soviet prisons and the labor camps of Siberia. Finding himself captive, he drew upon his strong faith in order to survive and reach out to fellow suffering prisoners. This volume is a powerful testimony of the horrors of concentration work camps and how God can work in extreme circumstances when He has a channel to work.

Ciszek's companion volume, "He Leadeth Me," is an internal, spiritual account of the same experience and period. Also highly recommended
I read it many years ago but it has left a powerful impression.