Five On A Treasure Island by Enid Blyton

The very first Famous Five adventure, featuring Julian, Dick, Anne, not forgetting tomboy George and her beloved dog, Timmy! There's a shipwreck off Kirrin Island! But where is the treasure? The Famous Five are on the trail - looking for clues - but they're not alone! Someone else has got the same idea. Time is running out for the Famous Five, who will follow the clues and get to the treasure first?