Why mermaids sing : a Sebastian St. Cyr mystery / C.S. Harris.

Enhanced descriptions from Syndetics:

It's September 1811, and someone is killing the wealthy young sons of London's most prominent families. Partially butchered, with strange objects stuffed into their mouths, their bodies are found dumped in public places at dawn. When the grisly remains of Alfred, Lord Stanton's eldest son are discovered in the Old Palace Yard beside the House of Lords, the local magistrate turns to Sebastian St. Cyr, Viscount Devlin, for help. Ranging from the gritty world of Thames-side docks to the luxurious drawing rooms of Mayfair, Sebastian finds himself confronting his most puzzling--and disturbing--case yet. With the help of his trusted allies--young servant Tom, Irish doctor Paul Gibson, and his lover Kat Boleyn--Sebastian struggles to decipher a cryptic set of clues that link the scion of a banking family to the son of a humble Kentish vicar. For as one killing follows another, Sebastian discovers he is confronting a murderer with both a method and a purpose to his ritualized killings, and that the key to it all may lie in the enigmatic stanzas of a haunting poem...and in a secret so dangerous that men are willing to sacrifice their own children to keep the truth from becoming known.