The unfamiliar setting in Moscow, in 1936, and the investigation of captain Korolev are so fascinating that I have read this book in less than 3 days.
I like a lot the very good mix between the History (The Stalin's purges, the denunciations, the self-criticism, the constant fear to be send to Siberia) and the investigation.
I learn quite a lot thanks to this book, mainly how USSR sold religious items to rich americains and the existence of quotas in denunciation of innocent citizens.

Alexeï Korolev and the majority of the characters are interesting as no one (but Larimine) are unidimensionals.
Korolev himself is still admiring Stalin, but hide a bible under his wooden floor, live in almost constant fear of saying something he shouldn't.

The relationship he has with the forensic doctor, his colleagues and the young woman with whom he shares an appartent are also interesting.
The chapter in which he meets Kolya, the leader of the Thief, is mesmerizing.

I'm looking forward reading the second book.