Of all the books Fiep Westendorp has illustrated, only a handful were written by Annie M.G. Schmidt. Yet the two women's work is inextricably linked. Fiep's individual and often humorous drawings complement Annie's writing perfectly. Up to Annie's death in 1995, Fiep illustrated dozens of her poems, columns and books.

Their collaboration lasted more than forty years and began in 1952 on the children's page of the newspaper Het Parool with the adventures of Yip and Yannika. The books came later. Fiep's illustrations helped make Tow-Truck Pluck and Odette classics of Dutch children's literature. Her work is immediately recognizable to all. Both text and illustration have proven timeless. Fifty years on, Fiep and Annie are still the twin mothers of the Netherlands' most famous and most popular pre-schoolers: Yip and Yannika.