Pelagia & the black monk - Boris Akunin

Just as the dust from the case of the White Bulldog begins to settle in the small Russian town of Zavolzhsk, it is shaken up once again by the arrival of a stranger: this time, a desperately frightened monk from the island monastery of New Ararat, who seeks the help of the bishop, Mitrofanii.
The monks have been troubled by visions of a dark, hooded figure: a figure that appears to walk on the waters of the vast Blue Lake surrounding their monastery and strikes terror into the hearts of all who encounter it. Sceptical of ghost stories and dismissive of rural superstitions, Mitrofanii dispatches Alexei Lentochkin, his clever young ward, to investigate the mystery, only for Lentochkin himself to appear to fall victim to the phantom. With sightings of the Black Monk occurring with disturbing frequency, and rumours of suspicious deaths reaching his ears, the Bishop decides to send two more of his most trusted advisors, in turn, to New Ararat, but they too meet with unexpected fates.
Finally, Sister Pelagia takes matters into her own hands, and, adopting a number of ingenious disguises, she ventures across the Blue Lake in search of answers, and in pursuit of the Black Monk. But as she delves deeper into the layers of secrecy that cloak the island and its strange population, and as the body count continues to rise, Pelagia begins to realise that an encounter with a ghost may be the least of her problems...