This is a rather literary historical mystery set in New York City in 1845 – the days of the Irish immigrants, fleeing from the potato famine. That was also the year NYC established a real police force.
The protagonist, a bartender named Timothy Wilde, is homeless and unemployed as the result of a large Manhattan fire which has also left him with a disfigured face. His brother, thanks to political connections, gets him a job on the brand new police force. His beat? The edge of Five Points, a seriously bad neighborhood – nightly murders for 15 years. See http://en.wikipedia.org/wiki/Five_Points,_Manhattan
On his beat Timothy meets and befriends a young girl whose nightgown is covered with blood. She tells him there are many, many bodies buried in a nearby forest area. Timothy rises to the occasion and gets immersed in a very difficult case of child prostitution, murder, corruption, religion and madness.