And Dangerous to Know (Henry Gamadge, 14)

Alice Dunbar was a very proper upper East Side woman with a very boring life. There is, in fact, absolutely no reason anyone can think of for Alice to go missing, and yet that’s exactly what she does, shortly after the death of an elderly aunt. Henry Gamadge thinks there’s more to the story, and as he tracks down Alice’s last trip, he turns up a secret life no one suspected. Originally published in 1949.