Holiday in Death (In Death, #7)

An Alternate Cover edition can be found here

No one likes to be alone during the holidays. For New York's most posh dating service, Personally Yours, it is the season to bring lonely hearts together. But Lieutenant Eve Dallas, on the trail of a ritualistic serial killer, has made a disturbing discovery: all of the victims have been traced to Personally Yours. As the murders continue, Eve enters into an elite world of people searching for their one true love--and a killer searching for his next victim. A world where the power of love leads men and women into the ultimate act of betrayal...