Five on a Hike Together by Enid Blyton

It's half-term and the Five are going hiking through the woods and up the hills. But when Anne and Dick take a wrong turn, it leads them all straight into danger! An escaped convict is on the loose and passes a strange message to Dick by mistake.

A stash of treasure has been hidden nearby -- will they discover it before the prisoner strikes again?

I loved the Famous Five books when I was a kid. I don't recall much about most of them anymore - though I do recall quite a bit about this one. The Five are on holiday (again!) and they chance upon a mysterious and puzzling message. It turns out to be a cryptic message and their adventure revolves around solving it and finding out the who and why of the message.

I'd recommend this series for kids who love adventure stories coupled with mystery and detective elements. It's like a younger version of the Hardy Boys or Nancy Drew.