Along Came a Spider (Alex Cross #1) by James Patterson

THE PSYCHOLOGICAL THRILLER THAT WILL HAUNT YOUR DREAMS 

He had always wanted to be famous. When he kidnapped two well-known rich kids, it was headline news. Then one of them was found – dead – and the whole nation was in uproar. 
   For such a high-profile case, they needed the top people – Alex Cross, a black detective with a PhD in psychology, and Jezzie Flanagan, an ambitious and beautiful Service agent – yet even they were no match for the killer. He had the unnerving ability to switch from blood-crazed madness to clear-eyed sanity in an instant. But was he the helpless victim of a multiple-personality disorder – or a brilliant, cold-blooded manipulator? 
   As he and the rest of the country watched Flanagan and Cross falling into his every trap, he knew he had made it – he was controlling the deadly game, and he still hadn’t made his most devastating move.