The Riddle of the Double Ring (Judy Bolton Mysteries) Reprint edition

Judy is surprised when Arthur Farringdon-Pett slips an engagement ring, a pigeon's blood ruby, on her finger. She forces Arthur to keep the engagement secret, as she still feels torn in her feelings between Arthur and Peter and needs more time to think. Meanwhile, Lorraine Lee guesses that Arthur has proposed to Judy. Lorraine decides to try to capture the thieves who robbed a fur store in Farringdon in an attempt to prove to Arthur that she can be just like Judy. When Lorraine disappears Judy and Arthur begin a desperate search for her, with no clue as to where she has gone. Judy and Arthur fly in Arthur's plane, which crashes. Arthur is hurt, and Judy knows what must be done, if only she can locate Lorraine. Judy's search for Lorraine also leads her to the fur thieves and an exciting confrontation.