Five Go To Demon's Rocks: Book 19 (Famous Five)

In book nineteen, the Famous Five travel to Demon's Rocks and discover a very old and valuable gold coin. Demon's Rocks is famous for its stories of bountiful treasure, but if the myth is reality, who does the treasure belong to?

 

The Five are determined to find out, but first they need to know who else is exploring Demon's Rocks for the treasure...