The State Counsellor - Boris Akunin

The reactionary Adjutant General Khrapov, newly appointed governor-general of Siberia and soon-to-be minister of the interior, is murdered in his official saloon carriage on his way from St. Petersburg to Moscow. The killer, disguised as Fandorin, boards the train at Klin, where it has been delayed by snowdrifts on the line. He leaves a knife bearing the initials "CG" (Combat Group) thrust up to the hilt in his victim's chest and escapes through the window of the carriage.
Fandorin is first arrested for the crime, but quickly released when the train pulls into Moscow and the General's staff realise their blunder. The murder threatens the career of Prince Dolgorukoi, Moscow's elderly governor-general, and Fandorin's greatest fan. He asks Fandorin to investigate, even though someone new, Prince Pozharsky, has been sent from St. Petersburg to head the investigation.
The head of the CG is a man named Mr. Green, the son of a Jewish pharmacist whose family suffered very badly from the pogroms. He bears a grudge and, together with his fellow revolutionaries, seeks vengeance. He is a man of steele, who has toughened himself so much that he is a worthy, and daunting, adversary.
However, this is a battle between good and evil and, Fandorin reminds us that two wrongs never make a right, and he battles against Green in a test of will and good versus evil. The mystery surrounds the mole: who is the leak? And what motivates them? This is what Fandorin must find out, even if it means that he places his own lover, the fiery Esfir, unders suspician. In the end, Fandorin becomes so disillusioned that he walks away from promotion, and to the life of a civilian, at last.