Trixie Belden #5 - The Mystery Off Glen Road - 1956
If you've read the first book in this series - The Secret Manison - you will remember that Jim left Trixie a diamond ring. Because of Trixie's "tomboy" nature, her parents are keeping the ring for her until she grows up. In The Mystery Off Glen Road, Trixie is trying to convince her parents she is grown up and feminine enough so they will give her the ring.
The real reason Trixie wants the diamond ring is so she can use it as collateral for her oldest brother, Brian - he is trying to buy his dream jalopy (car), but doesn't have the money for a downpayment. Trixie feels she and her brothers can earn the money, if they can convince the owner not to sell the car to someone else.
In another story line, the Wheeler's surly gamekeeper quits without notice. Trixie and Honey agree to take on the gamekeeper's duties - again to help Brian earn money for his car. But another mystery hits the Bob Whites, and they soon fear a poacher is loose in the Wheeler's game preserve.
This book really shows Trixie's determination in following through with a project. She cares more about helping her brother than doing things for herself. This is a lesson that siblings today could learn from.