"Karen's Leprechaun" (Baby-Sitters Little Sister #71) by Ann M. Martin

Spending spring break at a resort in the Caribbean, Karen explores caves, hunts for seashells, and goes snorkeling until she meets Sandy, a quiet and seemingly standoffish seven-year-old who has an unhappy secret.

The back cover of this paperback has gotten wrinkled from poor handling, but is otherwise in acceptable condition.