The Art of Detection by Laurie R. King
I'd forgotten that King writes the Mary Russell/Sherlock Holmes series until I finished reading this book. Throughout it, I kept thinking, "Boy, does she know her Holmes!" D'uh.
This is a Detective Kate Martinelli book. Kate and her partner Al Hawkin are assigned the murder of Philip Gilbert, who appeared to be obsessed with the fictional character of Sherlock Holmes. The main floor of his house was decorated in an 19th-century style that would not have been out of place in 221 Baker Street, including gas lamps and an icebox. An authority on Sherlock Holmes and Arthur Conan Doyle, he also had a valuable collection of his own.
Gilbert's body is discovered in an old gun battery on the side of a cliff in Golden Gate Park. When they find a manuscript that may have been written by Conan Doyle in his safe deposit box, and realize that his murder mimics the murder in the story, they begin to concentrate on the members of Sherlockian group to which he belonged.
This is an absorbing story, which one could easily believe is really Conan Doyle's work. Reading the story along with Kate really pulls the reader into the narrative, and it's actually disappointing to reach the end of the book.