Complex sentence: The book is unique in that there are no evil characters (not even Stalin himself) and, despite describing grim conditions, the author remains cool-headed and even humorous throughout, despite having endured the conditions in his own life.
Split into simpler sentences: The book is unique in that there are no evil characters (not even Stalin himself). Also, even while describing grim conditions, the author remains cool-headed and often humorous throughout, despite having endured the conditions in his own life.