My first experience with Georgette Heyer's detective novels was disappointing. Penhallow is, I think, widely regarded as a poor book. But Nigel Moss persuaded me to take another look at her, and recommended The Unfinished Clue (1934). He's a good judge - it's a light, entertaining mystery in the classic Golden Age vein.
The victim is not a blackmailer, but old General Billington-Smith is so utterly horrible that almost everyone who sets foot in his country house has cause to want him dead. And in due course he is found stabbed in his study. No great loss, frankly. Inspector Harding from Scotland Yard is called in by the Chief Constable, and proves to be a likeable character. He even falls in love with a member of the house party, who is sister-in-law of the deceased.
Dorothy L. Sayers reviewed this novel in the Sunday Times and pointed that it is full of cliches, but that it's written in such a lively way, with a nice line in comedy, that this doesn't matter. I agree. The 'unfinished clue' of the title is a 'dying message clue' scribbled by the General on a scrap of paper. I didn't find it difficult to figure out the meaning of the clue, although some aspects of the solution to the puzzle did elude me.
Heyer's strength was her dialogue rather than her plotting, and she excels at allowing her characters to forward the story through conversation (a contemporary writer who displays the same mastery of dialogue is Simon Brett). I liked this book, and thanks to Nigel's advocacy, I'll be happy to read more Heyer one of these days.