Book 9. The Sign of Four, by Sir Arthur Conan Doyle. Unabridged audio.
First published in1890, this is the second novel to feature Sherlock Holmes. It is a much more enjoyable read than A Study in Scarlet, which despite its historical value, gets bogged down for a long stretch. Although the revenge plot is similar to that in the first novel, this story moves a very nice pace, and both the plot elements and character beats are strong.
This book contains much of what a Holmesian mystery should contain. It has more of a historical fiction feel to it, with all of the action is confined to London. Holmes is in his element in this locale, whether he and Watson are using a dog to track a man through the streets of London, or the irregulars are scouring the ports for a very particular boat, or he is jousting with detective Athelney Jones.
The detective is hired by a beautiful young governess, Mary Morstan, to solve the mystery of the single pearl she receives every year on the anniversary of her father's death. Holmes reveals the complex plot, which involves a pact of four convicts and a pair of corrupt prison guards to smuggle a treasure out of India. After the case is solved, Watson proposes marriage to Mary, and the two wed.
The novel starts and ends with Holmes' cocaine use. This adds characterization to both Holmes and Watson, in his reaction to his friend's addiction. There is also a nice sense of symmetry here, by beginning and ending the story in the same fashion.
The very end of the novel is typical Holmes, with Watson asking what the great detective has gotten out of his involvement in the case. Holmes did all the work, while Watson found a wife, and Jones has received credit for the arrest.