Hound of the Baskervilles is the perfect Scooby Doo mystery. Scooby Doo plots start with a visit to a creepy secluded location. The gang finds that the locals are being terrified by a monster of some sort. After some ominous warnings that they should leave immediately, they have an encounter or two with the monster and then figure out that someone took advantage of a local legend in order to frighten everyone away so that they could claim some land or steal a fortune. That is precisely the plot of Hound of the Baskervilles. All that is missing is the villain saying "I would have gotten away with it too if it weren't for that meddling Sherlock Holmes!"
The second half of this collected edition is the Valley of Fear, the fourth and last of Doyle's Holmes novels. Doyle uses the same structure he used in Study in Scarlet, in which the first half of the novel has Holmes solve a murder and then the second half reveals the killer's elaborate backstory. Again like Study in Scarlet the backstory is set in the United States. This one involves a corrupt secret society that engages in extortion and murder. I suspect Doyle used this structure in order to write novels about characters and situations other than those of his Sherlock Holmes series and the wrap them in a Sherlock envelope to satisfy his fans.