This was a nostalgia read.  I first read this book 20 or so years ago when it came out and it still stands out as one of the best books I've ever read. 
 
Set in 1890s NYC, an alienist (or a psychologist in modern terms) and his friends set out to solve the murders of several "boy-whores" - children who dress as women and sell their bodies in debauched bars in the city. What I enjoyed is that so much of NYC's history is tied up in the story, from Teddy Roosevelt's stint as police commissioner to the landmarks still standing today to the city's notorious gangsters stirring up trouble. The plot is a taut thriller than builds slowly as the investigation goes from clue to clue to build a psychological profile of a killer and tracking him down.  And that's what I think appeals to me most; there's no "stumbling onto the killer" moment; the groundwork is laid, and that's what leads them to the killer.  It's a very well written mystery rich in detail without being dry and stilted. 
 
A great novel and I enjoyed revisiting the world of Kreizler, Moore and Sara.