Ghost Letters — Stephen Alter

When two modern-day kids discover a grotesque secret in an abandoned mailbox, they have no idea they are about to be drawn into a mystery that began on the other side of the world. Through the help of an English genie and a phantom postman, the two children begin to communicate with another boy, a young calligrapher's apprentice who lived 125 years ago in an Indian village.

Writing back and forth, across continents and centuries, the three children eventually realize the possibility of changing history by delivering three letters that were never received. If they can make sure these lost letters reach those for whom they were intended, love may be restored, the life of a kidnapped child could be saved, and a secret agent might be able to prevent a pointless war.