When he became 40 years old and was living in Chicago, Baum started writing children's books. These books were often based on the stories he had told to children over many years. After several successes in the late 1890s, Baum published what he called an "American fairy tale," "The Wonderful Wizard of Oz", in 1900. The book had pictures drawn by the American artist W. W. Denslow. Baum had to pay for the first "Oz" book himself. No Chicago publisher wanted to publish "The Wonderful Wizard of Oz". One man even told Baum that if people had wanted to read this kind of book, it would already have been written. 