The Black Book by James Patterson and David Ellis

A murder leads detective Billy Harney to uncover the secret of a brothel to Chicago's elite. There's a house full of incriminating evidence, but the one thing they really need is missing: the little black book.

I love being immersed in a story that completely sucks me in because of the mystery. You cannot be focused on the outside world while you're trying to unravel this plot. Highly recommended for mystery fans.