A very different offering from Georgette Heyer. She wrote it as a "contract breaker" and it evidently achieved its purpose. While it contains a romance, it is not a romance novel and anyone picking up this novel on the basis that Heyer is known as a romance novelist would be in for an unpleasant surprise. Heyer also wrote mysteries, but this is not one of them, even though it is about a murder. The murder does not occur until about 2/3 of the way through the novel and the who, why, and how are known from the time the murder occurs. Notwithstanding this, the novel is suspenseful. The reader spends the first 2/3 of the novel wondering which of the large cast of characters will actually commit the murder and the last third of the novel wondering whether the murderer will be detected. The novel's real strength lies in the character studies and its treatment of the psychological effect of an evil deed committed for a good reason. Not a novel to enjoy, as such, but an interesting read nevertheless.
ETA: I am currently reading Jennifer Kloester's recently published Georgette Heyer: Biography of a Bestseller and it turns out that my understanding that Penhallow was written as a contract breaker was incorrect. As it turned out, it was a contract breaker, because Hoddder & Stoughton refused to publish it, but Heyer was immensely proud of the book and at the time she wrote Penhallow considered it to be her best work.