Motherless Brooklyn by Jonathan Lethem (1999) Doubleday

Lionel Essrog, a man with Tourette’s, along with three other marginalized guys was recruited as teenagers by Frank Minna, who does questionable jobs for a couple of old gangsters.  His mentor went away for a few years with a brother.  When he came back, he created a car service as a front for a detective agency, which was really a front for doing what he always did.  Lionel is along as backup when his mentor is killed.  As a detective, even if that was a front, he feels obligated to bring the murderer to justice, in spite of his Tourettic tics.