The Sign of Four isn't much of a mystery, but it certainly is a great adventure. There's even a treasure hunt!
The murder of Bartholmew Sholto takes place in chapter five. Holmes announces the name of the murderer, Jonathan Small, in chapter six! There are no other suspects - except to Athelny Jones. There's a wonderful exchange between Holmes and Watson after Jones arrests Thaddeus Sholto along with a housekeeper, a butler, and gamekeeper:
"Isn't it gorgeous," said Holmes, grinning over his coffee cup. "What do you think of it?"(But the butler actually did do it, sort of.)
"I think that we have had ourselves a close shave of being arrested for the crime."
"So do I. I wouldn't answer for our safety now, if he should happen to have another of his attacks of energy."
Shortly thereafter follows the thrilling chase on the Thames - adventure, but not mystery.
I re-read The Sign of Four last week in the splendid Sherlock Holmes Reference Library edition from Gasogene Books, edited and annotated by Leslie S. Klinger. I happened to have re-read The Maltese Falcon immediately beforehand. The two detective novels (and detectives) are very different in type and tone. But it struck me that they are both about a treasure which is ultimately lost.
Ah, the Agra Treasure - "That's the stuff that dreams are made."