''Everybody Always Tells'' by E. R. Punshon (d. 1956), public domain. This classic Golden Age whodunit revolves around a locked-room murder, a stolen necklace, and a prominent scientist's fatal stabbing.
The narrative kicks off when Scotland Yard's Bobby Owen and his wife Olive are bargain-hunting in a London department store. The trip turns bizarre when Olive discovers a pearl necklace stuffed into her handbag, secretly placed there by Lord Newdagonby. When his Lordship denies the act, events spiral as a prominent scientist and patent agent is found dead in a locked room. Bobby Owen must unravel the mystery, connecting the necklace, a dilettante inventor, and a disrespectful daughter to solve the baffling locked-room crime.