The first of the "Magic Tree House" books, Dinosaurs Before Dark, was published in 1992. She was initially inspired to write the books while working in a teen homeless shelter and realizing that for the teens writing themselves into stories taking place in the Himalayas or Serengeti had a major effect on the teens. Osborne says she tried writing the book seven different ways before finding a way that worked." It introduces the main characters of Jack and Annie, a brother and sister duo of adventurers who are transported to different areas of time thanks to the titular magic treehouse. The first book established the format for feature books and introduced recurring characters Morgan le Fay and Merlin, as part of the Arthurian motifs. Osborne says she is more like Jack but wishes she was more like Annie.