Harriet The Spy is a children's novel written by Louise Fitzhugh, first published in 1964. The story follows a young girl named Harriet M. Welsch, who aspires to be a writer and spends her time observing the people around her. She keeps detailed notes about her friends and family in a notebook, which she believes will help her understand the world better. However, Harriet's life takes a turn when her notebook is discovered, leading to conflicts with her peers. The book explores themes of friendship, betrayal, and the challenges of growing up, making it a classic in children's literature.