Walking Shadow by Robert B. Parker

A murder draws Boston PI Spenser into the dramatic world of theater in this New York Times bestseller in Robert B. Parker's long-running series.
In a shabby waterfront town, an actor is shot dead onstage. Granted, the script left much to be desired. But there's more behind the scenes than an overzealous critic-and Spenser and Hawk are combing Port City's underworld to find it...
"Great fun...[Spenser] is still the cockiest and wittiest P.I. on the block."-The New York Times