Setske de Haan, better known by her pen name Cissy van Marxveldt, was a Dutch writer of children's books. She is best known for her series of Joop ter Heul novels.
Setske de Haan was born on 24 November 1889 in Oranjewoud, a village in the northern province of Friesland in the Netherlands. She was the daughter of IJnze de Haan, a headmaster and history teacher, and Froukje de Groot.
Cissy van Marxvelt embarked on her literary career by writing articles and stories for Dutch magazines. In 1916, she published the first book in what was to become a series of novels about a headstrong girl, Joop ter Heul. The books, similar in theme to Louisa May Alcott's Little Women, contain many diary entries and letters. They chart the fortunes of Joop, her sister and her school friends, from girlhood through marriage.
Van Marxvelt's Joop ter Heul novels had a notable influence on the writings of Anne Frank, who addressed her diary letters to an imaginary friend named Kitty. Anne Frank scholars, as well as Anne's friend Kitty Egyedi, are united in their belief that Frank's Kitty was based on a character created by Van Marxveldt: Kitty Francken, a friend of Joop's and a frequent recipient of her letters.
Van Marxveldt also wrote many other young-adult books, of which Een zomerzotheid ("A Summer Folly") was a particular best-seller, which made her affluent.
In 1914, she met Leo Beek, who was a department store manager as well as a reserve infantry officer. De Haan and Beek married on 2 February 1916 and had two sons, Leo and IJnze. During the German occupation of the Netherlands, Beek was a resistance fighter. He was arrested and later executed in the Westerbork transit camp in 1944, though it was 1946 before De Haan learned of his fate. She dedicated her last book She Suffered Too to him. She died in Bussum on 31 October 1948, aged 58.
On 24 November 2007, the planetoid (10667) Van Marxveldt was named after her. It orbits between the planets Mars and Jupiter.