The White Cottage Mystery by Margery Allingham
The Exceptional! Took the first few chapters to get back into that British detective, cozy 
village/cottage sensibility and mode, but then really enjoyed the story. Sort of an Orient Express vibe to it as the cast of characters all had excellent reasons to dislike, nay, loathe Mr. Crowther. And a loathesome man he was, although today we might not be so shocked and awed by his manipulations and mental cruelty.
After the first chapters as I was settling in for a great read, I also looked up Ms. Allingham. The fact that she was J.K. Rowlings favorite read was incidential, what I loved was that this first book was originally serialized and that made the chapter structure so much more interesting and beautiful, the handling of each of the main characters, the retuning to them when unable due to timing, proximity or alibi made them an unlikely killer. How much fun to be a reader at that time where you had to wait for the next installment!
And when W.T wrote to Jeffery that he had abandoned the case I felt sure he had found the killer and yet this was one of the few times I was actually baffled as to who was the killer in this story. Masterfully done! The reveal was an Aha! Moment for me and I suddenly saw all the clues line up. Delicious. I shall read more of Ms. Allingham, definitely.