Five on a Hike Together

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.