Sherlock Holmes, The Annotated:

The Annotated Sherlock Holmes by William S. Baring-Gould is one of my most prized possessions. I have the single volume edition and I’m sure someone somewhere has written a mystery where this book has been the blunt instrument. Sadly it is out of print (Update: The two-volume set is again available for new from Amazon for $150), although you can find used copies for sale at Barnes & Noble and Amazon or your local antiquarian bookshop (a shout out to The Hermitage in Denver).