The Sign of Four (1890) was the second volume in the Sherlock Holmes series, written by Arthur Conan Doyle three years after the first. So, one could not say that the demand for a sequel was overwhelming. Curiously enough, at the evening party the offer to publish such a sequel was made to Conan Doyle, the same magazine editor made an offer to Oscar Wilde for The Picture of Dorian Gray. That must have been a fairly interesting periodical.
The Sign of Four opens with the revelation that Holmes was injecting himself three times a day with cocaine or morphine, depending upon the desired effect. Holmes explained he needed the cocaine for mental stimulation when he was not busy with a challenging case; I suppose he needed the morphine to smooth out the end of the cocaine high and sleep. This is a non-typical twist for the central character of a Victorian magazine series; I'm surprised that such a character was not roundly condemned to obscurity at the time.
In addition, it was made clear in A Study in Scarlet that Holmes was what Germans call a Fachidiot, a person who is an expert in their specialty but knows nothing about anything else. I've known a good number of them in my professional life. Some of these Fachidioten are even dismissive of all other knowledge, as Holmes showed himself to be when he pooh-poohed the fact that the earth revolves about the sun in a solar system. Indeed, he proclaimed that he would do his best to forget that irrelevant fact, since it was of no use to him in solving crimes. (!) (And yet, Conan Doyle had him speaking about miracle plays, Theravada Buddhism, medieval pottery, etc. at dinner in this book. I guess consistency is the hobgoblin of little minds...) Perhaps such a character weakness, which tends to optimize a certain kind of success, was viewed positively at the time?(*)
But for a less morally bound reader these twists to Holmes' character offer the novelist some edges to play with.
So, three years after the events related in A Study in Scarlet Watson and Holmes are still bachelors sharing the three room flat on Baker Street, and Holmes' career as a "consulting detective" is going well; now he is also consulting on cases on the European continent. And Holmes is dissatisfied that Watson had written up A Study in Scarlet, for Watson had, according to Holmes, overly romanticized the proceedings and did not sufficiently concentrate on the details of Holmes' techniques and reasoning skills. Watson complained, sotto voce, that these complaints were due to Holmes' overweening egotism and vanity.
Let's face it, there is already more grist for the mill here than is to be found in most detective series. On top of that are Holmes' rather unique forensic skills. Then there is the late Victorian London setting with the delicious English in use at the time (so different from early 21st century Global English with its mixture of technocratese and a slang which can express very little more than approval or disapproval (**)), and all one needs is a good story to pass a most pleasant few hours.
As a detective story, the case described in A Study in Scarlet was not particularly interesting to me; the interest lay elsewhere, as I describe in my review:
But in The Sign of Four the intrigue is much more involving. There is no way I'm going to let slip a crucial spoiler in a mystery, but this intrigue originates in the Andaman Islands, involves a young woman Watson is gaga about, a vast fortune of jewels, an exciting boat chase, and murder most foul...
It's amusing to me to see what Conan Doyle finds worthy of verisimilitude and that which he doesn't. For example, he exerts quite a bit of trouble to go on about the difference between different types of ash, but pretends that after living for three years together in close quarters Watson is not already aware of the little monographs Holmes has written (in fact, Watson mentioned the one on ash in A Study of Scarlet). Actually, he does this to trigger an info dump, which is rather lame of him.
But Conan Doyle is beginning to hit his stride in this book. After this book there was a big demand for Sherlock Holmes stories. Recommended.
(*) As a side note, I have a lovely colleague who is now 76 years old and refuses to retire. He says that he has "no outside interests". I answer, but start reading around and find some outside interests. "Reading is boring," is the response. I love the guy, but WOW! And he is a university professor...
(**) I mean, do you know, like, what I mean?