A Three Book Problem

It's a crisp, early October weekend, and business is slowing down as fall descends at the Sherlock Holmes Bookshop and Emporium and adjacent Mrs. Hudson's Tea Room. Wealthy philanthropist and prominent Sherlockian David Masterson has rented Suffolk Gardens House, where he plans to entertain his friends in a traditional English country house weekend. Before Gemma can ponder their relationships a poisoned dart sails through the window of the library, presenting Gemma Doyle with a three-book problem.