Michael and Mollie Hardwick’s indispensable and affectionate “Sherlock Holmes Companion” gathers together all the various elements in the stories to provide a unique reference source. Included is a comprehensive Who’s Who of characters, a concise summary of the plot of each story, detailed biographies of both Holmes and Watson, an anthology of their often witty aphorisms, and an essay on the author’s life and relation to his immortal and much-loved characters.