The Sign of Four by Sir Arthur Conan Doyle
A couple of years after introducing the world to the detective on Baker Street in A Study in Scarlet, Doyle followed up with The Sign of Four. In it Holmes and Watson are solicited by a woman named Mary Morstan, whose father died under unusual circumstances years ago and who for the past several years has been receiving an anonymous gift in the mail every year - a rare and beautiful pearl.
As the pair investigate both the death of Miss Morstan's father as well as the origins of the pearls, they discover that the pearls are part of a much larger treasure, a treasure that was discovered and hidden years ago by four men. Among those four men were Miss Morstan's father and the father of the man sending the pearls. Once again Sherlock Holmes uses his remarkable powers of observation and deduction to solve the mystery surrounding the four men and the treasure.
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