The Mystery Books are by far my favourite. They are about Five Children and a dog, generally known as "The Five Findouters" by the youngest of the group Bets.
They try to solve Mysteries in the village of Peterswood, and pit their wits against the local bobby Mr. Goon
The cleverest of the Group is Frederick Trottville known as "Fatty" who wants to be a Detective when he grows up, he even becomes a Master of Disguise, dressing up as a Butcher Boy, Mail Boy, An Old Man etc.
I have read these over and over again and even though i am now a "middle aged man" I never tire of them.
I also own 1st Editions of all the Mystery books. If you need them in please pm me and i will send you over my own link to these and the "Five" Series in epub format