Five Go Down to the Sea by Enid Blyton

On holiday in Cornwall, the Five encounter strange lights at night and tales of wreckers—men who lured ships to their doom. Curious and suspicious, they investigate the rocky coast and uncover clues pointing to criminal activity. Their search leads to secret caves, old legends, and a dangerous smuggling operation. As always, their bravery and clever thinking bring the mystery to light. This adventure has a strong sense of place and atmosphere.