Based on Christophers Deduction, who wrote this letter

Chris notices a letter addressed to him. He takes the letter and quietly leaves his father's room, hides the letter under his mattress, and goes downstairs to have dinner with his father.

When he goes back up to his room he opens the letter. It is from his mother, writing from London. It is dated a year and a half after she died.

Christopher says that it must be a letter to another Christopher from his mother, and that he was in the middle of another mystery.

Idk