Enid Blyton 
Five Go Off in a Caravan [EPUB ebook] 
Famous Five #5

Just when the Five were wondering what to do for their summer holiday, a circus with all its gay caravans and performing animals came along the road. This gave them the thrilling idea of going off in a caravan themselves. And thrilling they found it, for when at last they caught up with the circus it was plain that some of the characters had more sinister ways of passing their time than clowning in the Big Top.

Without Timmy and Pongo the chimpanzee the criminals would never have got their deserts, and even Anne, who had wanted an ordinary holiday, not too exciting, was sorry when their adventure was all over.

This is the Five’s fifth adventure.