Harriet the Spy:

Harriet the Spy is a 1964 novel for children by Louise Fitzhugh that was made into a movie starring Michelle Trachtenberg in 1996. 

"Harriet M. Welsch is an outgoing 11-year-old girl aspiring to be a writer and a spy," says Wikipedia.  "Harriet lives a privileged life on the Upper East Side, something she reflects on throughout the book. As practice for her future career, she observes others carefully and writes everything she thinks in a notebook. Her nurse, Catherine Golly (known to Harriet as Ole Golly), has encouraged this. It is shown later in the book that Harriet has become so used to writing things down that she cannot think properly without a notebook."