"Harriet the Spy" is a children's novel written by Louise Fitzhugh, first published in 1964. The story follows Harriet M. Welsch, a young girl who aspires to be a writer and spends her time observing the people around her. She keeps her observations in a notebook, which she uses to document her thoughts and feelings about her friends and family. When Harriet's notebook is discovered, it leads to conflicts with her peers and challenges her relationships. The book explores themes of friendship, honesty, and the complexities of growing up, making it a beloved classic in children's literature.