I loved the setting of this and the bit of twist I had not really expected. I always love it when Holmes goes off on another subject as well as the obvious ( )

In this story, Holmes and Dr. Watson find themselves in Cornwall one spring for the former’s health, but the holiday ends with a bizarre event. Mr. Mortimer Tregennis, a local gentleman, and Mr. Roundhay, the local vicar, come to Holmes to report that Tregennis’s two brothers have gone insane, and his sister has died. Tregennis had gone to visit them in their village (Tredannick Wollas), played whist with them, and then left. When he came back in the morning, he found them still sitting in their places at the table, the brothers, George and Owen, laughing and singing, and the sister, Brenda, dead. The housekeeper had discovered them in this state, and fainted. The vicar has not been to see them yet. Tregennis says that he remembers one brother looking through the window, and then he himself turned to see some "movement" outside. He declares that the horrific event is the work of the devil. Mortimer Tregennis was once estranged from his siblings by the matter of dividing the proceeds from the sale of the family business, but he insists that all was forgiven, although he still lives apart from them. The doctor who was summoned, reckoned that she had been dead for six hours. He also collapsed into a chair for a while after arriving.
Holmes goes to the house in question and, apparently carelessly, kicks over a watering pot, soaking everyone’s feet. The housekeeper tells Holmes that she heard nothing in the night, and that the family had been particularly happy and prosperous lately. Holmes observes the remains of a fire in the fireplace. Tregennis explains that it was a cold, damp night.
Afterwards, Holmes lays the case out to Watson thus:

Quite obviously, there is no point in attributing the tragedy to the Devil; therefore, what took place can only be the work of a person.
Whatever happened to those people happened right after Tregennis left, for they had not moved and everything was in the same place;
Mortimer Tregennis went swiftly back to the vicarage where he lives (a footprint sample was obtained in the watering pot “accident”);
The only suggestion of an explanation — the "movement" — comes from Mortimer Tregennis;
Given the weather, anyone appearing at the window and doing something horrifying enough to instantly kill someone would have had to come right up to the window thus trampling the flowerbed, which is still intact;
What on earth could this person at the window have done to cause such horror?
None of this seems to make for an elementary case, but soon, new questions are raised.
Dr. Leon Sterndale, the famous hunter and explorer, aborts his sailing from Plymouth after the vicar wired him (as the Tregennises are Sterndale cousins) with the tragic news. He asks Holmes what his suspicions are, and is displeased when Holmes will not voice them. After Stermdale leaves, Holmes follows him discreetly.
The morning after Holmes comes back to his room, apparently none the wiser for following Sterndale, the vicar arrives in a panic with the news that Mortimer Tregennis has now died in the same way as his sister. The two men, along with Watson, rush to Mortimer’s room, and find it foul and stuffy, even though the window has been opened. A lamp is burning on the table beside the dead man. Holmes rushes about, examining many things. The upstairs window seems especially interesting. He also scrapes some ashes out of the lamp, and puts them in an envelope.
Holmes deduces how the victims died or went mad and why people present when the death rooms were first opened fainted or felt unwell in each case. He tests his hypothesis by buying a lamp like the one in Tregennis’s room, lighting it, and putting some of the collected "ashes" on the smoke guard. The smoke from this powder is so potent a poison that Holmes is immediately struck down. Watson is able to resist and drags Holmes out of the room just in time.
It is clear to Holmes that Mortimer Tregennis poisoned his siblings, but who killed Mortimer?
It is Dr. Sterndale, who left physical evidence at the vicarage clearly implicating himself. Holmes confronts Sterndale, who explains that he loved Brenda for years (but had been unable to marry her because of the current marriage laws which prevented him from divorcing his wife even though she abandoned him years ago) and killed Mortimer in revenge for the cruel murder.
The poison is called Radix pedis diaboli (“Devil’s-foot root” in Latin), Sterndale collected from Africa as a curiosity. The toxic contents of the plant root are vaporized by heat and diffuse into the local atmosphere. He once explained to Mortimer what it was and what it was capable of, who then stole some to murder his siblings by throwing it on the fire just before he left. Mortimer thought Sterndale would be at sea before news reached Plymouth, but Sterndale recognized the poison’s effects from the vicar’s description of the tragedy and deduced right away what had happened.
Holmes’s sympathies in this matter lie with Sterndale, and he tells him to go back to his work in Africa.
I recommend this book to the permanent library of all readers that enjoy a well written mystery story. They will not be disappointed! ( )