Looking for a mystery / thriller with smart or witty male protagonist that is not in law enforcement.

I've always been fond of stories where the main character comes from a different field to help solve a case or mystery. Characters like Robert Langdon from Da Vinci Code, or even someone like Richard Castle from the TV show Castle.

Its fine if they help the police/FBI/CIA, but its more that they don't come from that world and bring a unique perspective in solving the mystery.