For the non-fiction category Historical/Political, jammies' Fahrenheit 451 would like to pick
The Gulag Archipelago by
Aleksandr Solzhenitsyn.
As an indictment of a system which caused unimaginable suffering in the name of "progress", this book is perhaps unparalleled. Drawn from the author's own experiences in the Gulag as well as the stories of others within the system, the three volumes of the
Gulag Archipelago show the reader the horrific consequences of Stalin's maxim: "The death of one man is a tragedy, the death of millions is a statistic".
Solzhenitsyn himself is not a particularly sympathetic character, and the books are definitely not light reading, yet I think they are and were important, even in an era where Stalinist Communism has been discredited (other than in North Korea), for the world they describe is the one where "the ends justify the means" has been taken to its furthest extreme and the inhabitants of that world are much the same whether Stalinist, nationalist, fundamentalist, or proponents of any other creed where the "elect" are separated from the "other".
PS: I'd never read Flashman before but I picked up "Flashman and the Angel of the Lord" due to the pick here and must say I was not disappointed. Great stuff!