How does Christopher find out his mom is still alive?

Christopher discovers a letter written to him by his mother over a year after her death. He doesn't quite understand his discovery, and even believes that the letter must be for another boy named Christopher.

In Section Six, Christopher does some investigating while his father is out of the house. He knows there's something in his father's shirt-box, but he doesn't expect to find forty-three letter addressed to him from his mother.