NY: Simon Pulse. 2003.
Jack's a fairy. That's the premise of the Buffy the Vampire Slayer novel Blood and Fog. To be specific, Jack is an evil fairy trapped between dimensions by the good fairies and who occasionally escapes his penitentiary plane in a swirl of ghostly fog to murder prostitutes. Should he ever succeed in killing enough, he will open the floodgates to his dimension and fairy hell will rein on earth.
This book was bad. Even if one could excuse the plot (which I could not), no one could excuse the lack of editing and glaring spelling errors. Not a recommended read to even the most devout of Ripper or Buffy enthusiasts.