The second Sherlock Holmes novel, and one of the most well-known stories now, The Sign of Four was still only moderately successful in its day, the more well-known short stories yet to be written, but it still holds an important place in Holmes folklore as it introduces Dr Watson’s wife and frames his later, more distant relationship to Holmes.
I’m not a huge fan of The Sign Of Four – for me it’s the weakest Sherlock Holmes novel. Firstly as it has a plot which may have been original at the time, but which now consists of little more than a series of mystery story cliches. This is probably not Conan Doyle’s fault, but what absolutely is his fault is the fairly shocking racism in the descriptions of a man from the Andaman Islands – he is depicted as nothing more than a hideous non-human savage. Interestingly this is not mentioned at all on the page’s wikipedia entry, while the page on A Study In Scarlet has an entire section on its controversial “Depiction of Mormonism.” The only conclusion I can draw from this is that we haven’t changed since 1890, or at least not as much as we would like to think.