Here is an excerpt from this online Fitzhugh bio:
In the late 1950s she and a friend, Sandra Scoppetone, began work on a beatnik parody of Kay Thompson’s Eloise, which was published in 1961 as Suzuki Beane. In 1964 she published her first novel, Harriet the Spy. Although it received mixed reviews from adults at the time, today it is widely regarded as a forerunner to the sort of realistic children’s fiction that would dominate the late 1960s and 1970s. Two novels about Harriet’s friends followed: The Long Secret in 1965 and Sport, published posthumously in 1979. At around the time she wrote Harriet the Spy, Fitzhugh also wrote a novel about two adolescent girls who fall in love, called Amelia; unfortunately her agent refused to take it on and the manuscript has since been lost.
Contemporary social issues figured prominently in much of Fitzhugh’s work for children: Bang Bang You’re Dead was a 1969 picture book with a strong anti-war message and Nobody’s Family Is Going to Change (1975) explored both women’s rights and children’s rights. Ironically, it became the basis of the Broadway musical The Tap Dance Kid with the book’s minor male characters taking a lead role, thereby completely overshadowing Emma, the female protagonist. Needless to say, this happened after Fitzhugh’s untimely death in 1974 at the age of 46.