The Ring O'Bells Mystery
by Enid Blyton
This is the third book in the Barney Mysteries (sometimes called the R Mysteries), a short series of six books. It follows the adventures of the siblings Roger and Diana, their younger cousin Snubby, and their nomadic friend, Barney. They are helped in their adventures by Snubby's spaniel, Loony, and Barney's Monkey, Miranda.
The children should be heading back to school, but they come down with the flu, and are sent to the small village of Ring O'Bells to recuperate for a week. While there, they get a message from Barney, saying that he would like to visit, and they quickly write to invite him to stay with them. Meanwhile, they have been exploring the village, and learn about the old house, Ring O'Bell Hall, that has a secret tunnel, secret hiding spots, and a tale of bells ringing mysteriously when enemies are near.
When Barney arrives, the children learn that the woman they are staying with is deathly afraid of monkeys (a fact that had not come up in casual conversation), and so Barney sneaks into Ring O'Bell Hall to spend the night. While there, he sees some odd goings-on, and the children start to investigate.
This is one of my favorite Blyton books; unfortunately, this was not one of her more popular series, and is fairly hard to find in America. This is a fun adventure involving secret rooms and passages, a gang of kidnappers, and a slightly spooky setting. The story has a minor theme of mirroring fairy tales (they meet women they call 'Old Red Riding Hood' and 'Mother Hubbard', and part of the adventure is set in a woodland cottage), but not to the point of being especially twee. It also mostly avoids being politically incorrect, aside from the fact that Diana is a wilting heroine who voluntarily skips all of the exciting bits of the adventure because girls are like that, obviously.
This book can stand on its own, and I'd recommend grabbing it up if you ever come across it. The first book in the series, if you are lucky enough to find it, is The Rockingdown Mystery; the second, immediately preceding The Ring O'Bells Mystery is The Rilloby Fair Mystery, and the next one is The Rubadub Mystery.