I've been rereading the golden-age British mysteries of Josephine Tey. Much better written novels than Agatha Christie's and less high-falutin' than Dorothy Sayers's can get, though I love her Lord Peter Whimsey. Tey wrote beautiful books - with the cleverest slip-in-the-backstory I've seen in Bratt Farrar.
My all-time top favorite - Bratt Farrar. A tale of impersonation and mystery... Is there a murder? Which is the evil twin? Very English. With horses. A wonderful novel. Next in my affections come the other stand-alones: Miss Pym Disposes, The Franchise Affair, then any with her regular detective. Her theory was that real evil-doers (not the exasperated spouse who bops their beloved with a bottle, but real evil) are motivated - or freed perhaps? unchained? - by a grossly swollen vanity, a no-one-matters-but-me ego. The definition of a sociopath. Having read the horrifying real-crime detection book The Murder Room, I suspect she has a point.
Readers interested in the controversy of Richard III will find The Daughter of Time fascinating.