The Castle of Adventure by Enid Blyton
This series by famed author Enid Blyton, set in the 1940s, tells the adventures of a group of four children and their pet parrot, aided by their adult friend Bill. In this title, the children are on vacation in the countryside near an old castle that is supposedly abandoned, but they stumble into a mystery when exploring the castle. Enid Blyton’s books are old-fashioned for today’s children, but they are full of excitement and adventure so I think their appeal is timeless. I especially love the Famous Five and Adventurous Four series.