Dr. Priestly is a series "detective" (a scientist called on by poor baffled detectives) in early 20th c England. I read an early Dr. Priestly, and found him horribly superior and inhuman; his creator obviously has made him a bit more approachable.
Two major crimes are reported to Scotland Yard: a jeweller is missing, along with the diamonds he carried; an unidentifiable body turns up in a rural village. Two detectives uncover various clues, then reach an impasse, until Dr. Priestly sets them in the right direction.
The plot is as complicated as usual for these early mysteries, but this one has a very clever criminal, flashes of humour, and the reappearance of a supposed dead body.