A to Z Mysteries: The Empty Envelope

E is for Envelope...Dink keeps getting envelopes in the mail addressed to
"D. Duncan." But the letters inside all start with "Dear Doris" and don't make
any sense! Josh and Ruth Rose think someone's playing a trick on Dink. But Dink
is sure there's more to it than that, especially after Doris herself shows up
and demands her letters!