St. Patrick's Secret (Sweet Petite Mysteries, #3)

When seventeen-year-old Sean visits his eccentric Irish grandfather, he discovers a secret that his Gramps is obsessed with—finding the family's cache of gold coins, stolen, according to his grandfather, by leprechauns. Though Sean doesn't believe in elves or leprechaun's he decides to spend his last summer before college with his grandfather, and joins him in his quest to find a purloined pot of gold.