The Triple Hoax — Carolyn Keene

When Nancy's aunt's friend is swindled out of a sizable sum of money, she invites Nancy, Bess, and George to New York to help figure out who is behind the theft. There, the girls see a performance of a magicians' group who stun their audiences with clever sleight-of-hand tricks. Because the magicians temporarily remove people's wallets and handbags, Nancy feels the actors aren't above suspicion. But will her hunch lead them to solve the case?