By modern, do you mean Soviet? Cos, then I have some great recommendations!

Gulag - Anne Applebaum (Also read Gulag Voices) - I LOVE this book, it was brilliant

The Gulag Archipelago - Aleksandr Solzhenitsyn - kind of a rough read.. still havent been able to finish it. Its massive.

The Whisperers - Orlando Figes - Reading this right now. Pretty good.