I, Alex Cross (Alex Cross #16) by James Patterson

Can Alex Cross survive his most chilling - and personal - case ever? Pulled out of a family celebration, Detective Alex Cross gets awful news: A beloved relative has been found brutally murdered. Vowing to catch the killer, he quickly learns that she was mixed up in one of Washington, D.C.'s wildest scenes. And she was not this killer's only victim . . .
The hunt for the murderer leads Alex and his girlfriend, Detective Brianna Stone, to a place where every fantasy is possible, if you have the credentials to get in. Soon they confront some very important, very protected, and very dangerous people who will do anything to keep their secrets safe. As Alex closes in on the killer, he discovers evidence that points to the unimaginable - a revelation that could rock the entire world.