The Greek Symbol Mystery — Carolyn Keene

When Nancy is told that a large inheritance from a Greek tycoon, meant for her friend Helen Nicholas, was stolen, she knows it's up to her to find the culprit. But after following clues in the form of a poisonous snake in a basket of apples and a strange symbol stamped on a rare Byzantine mask, Nancy and her friends find themselves in the middle of a ring of art smugglers. Will they discover the secret of the Greek symbol and recover Helen's inheritance?