Where All Paths Meet by Gregory Ashe

Holmeses. Moriartys. Adlers. What's a Watson to do?
When Jack Moreno receives an anonymous invitation to the Zodiac anniversary gala, he's inclined to ignore it. His best friend (and more), Holloway Holmes, vanished from his life five months ago, and Jack has no desire to bump into Holloway's terrifying father, Blackfriar. But included with the invitation is an offer Jack can't refuse: the promise that he'll learn what really happened to his mother.
At the gala, an anonymous text sends Jack on a collision course with an amateur thief and, more importantly, with Holmes. Together, they must race to recover what the thief has stolen—information not only about Jack's mom, but about the Holmes family. Information that could bring down Blackfriar's empire.
By the time they reach the thief, though, she's dead, and the stolen information is missing. To recover it, Jack and Holmes will have to learn who killed her. But other people, powerful people, are searching for the information too, and someone has already tried to kill Jack once. He'll need luck and quick thinking to stay alive. And, of course, he needs Holloway Holmes.