This paper describes an approach for developing adaptive electronic textbooks and presents InterBook, an authoring tool based on this approach which simplifies the development of adaptive electronic textbooks on the World Wide Web. The following topics are discussed: (1) content structuring, including the domain and user models, the glossary, and the annotated textbook; (2) functionality, including advanced navigation, adaptive navigation support, and prerequisite-based help; (3) authoring with InterBook, including a typical scenario for using this tool; and (4) indexing for more intelligent authoring on the Web. (Contains 15 references.) (DLS)