I Know a Trick Worth Two of That — Donald E. Westlake

Sam Holt spent years playing Packard, everyone's favorite TV-detective; he's got no desire to play sleuth in real life. But when a long-lost pal calls with a rant about conspiracies, sinister cargo ships and hit men, and then gets poisoned at a party full of Holt's closest friends, there doesn't seem to be a lot of choice. Sure, Holt could leave it to the cops and cross his fingers. But Packard would never have taken such a weenie's way out. And Holt, to his astonishment, finds that he can't take it, either.