In have a couple of nominations.
Brat Farrar by Josephine Tey, first published in 1949.
Amazon
Also there's a PD version at the Unversity of Adelaide's website. You can read it online:
https://ebooks.adelaide.edu.au/t/tey/josephine/
The other book I'd like to nominate is
Full Dark House by Christopher Fowler, the first of his Bryant and May series.
Spoiler:
This mystery features the impending retirement of a Scotland Yard detective and the death of another. When Arthur Bryant is apparently blown up, his erstwhile partner, John May, begins reflecting on their first case together more than 60 years earlier. May, a raw recruit of 19, and Bryant, a 23-year-old detective, became the core of the Peculiar Crimes Unit, created to handle cases that were too important to ignore, yet that somehow seemed disproportionately insignificant in the face of the hundreds of civilians killed each night during the Blitz. Both men had been hurried through training and were suddenly faced with the strange case of the Palace Phantom, a killer victimizing the cast in an elaborate production of Offenbach's Orpheus in the Underworld. May was both intrigued by and dismayed at Bryant's methods and seeming flights of fancy. He used everything from crime-scene forensics to spiritualists to help him build his case. Fowler skillfully shifts the action between 1940 and the 21st century, building suspense and growing awareness as each case comes to its respective climax. Not surprisingly, they are connected. The details of wartime London and the destruction and deprivation of daily life are vividly conveyed. Today's teens will identify with the young lives so drastically affected by the war while following the clues, and red herrings, to a satisfactory conclusion
Amazon
Kobo
Amazon UK