Final answer:
Nelly discovers Catherine's hidden letter tucked inside a book in Chapter 21 of 'Wuthering Heights'.
Step-by-step explanation:
In Chapter 21 of Emily Brontë's Wuthering Heights, Nelly discovers Catherine's hidden letter tucked inside a book. While cleaning, she finds the letter when it drops from between the pages of a volume that she had taken down to dust. This reflects a common motif in gothic literature where characters often find old, significant documents hidden away in books, which then reveal secrets or add depth to the narrative.