What deception of Catherine's does Nelly discover in Chapter 21? How does Nelly handle the matter?

Nelly learns that Catherine and Linton have started a secret correspondence, and Catherine is hiding his letters in the study. Nelly threatens to show the letters to Mr. Linton if Catherine does not stop and Catherine sadly agrees. Nelly sends a note to Wuthering Heights, saying that no more letters will be exchanged or accepted.