Several countries are trying to provide access to computing education for all secondary students. However, there are not enough teachers who are prepared to teach computer science. Interactive electronic books (ebooks) are a promising approach for providing low-cost professional development in computer science. Over the last four years, our research group has been conducting design-based research by iteratively developing and testing versions of a teacher ebook to help secondary teachers with no programming experience learn to teach an introductory programming course. The interactive elements in the ebook were designed based on research results from educational psychology and are intended to make learning more efficient and effective. Our goals for this effort are to increase teachers' knowledge of computer science concepts and to improve teachers' confidence in their ability to teach computer science. In this paper we summarize our previous work and report on a large-scale study of version two of the teacher ebook. We also recommend several design principles for interactive ebooks for computing teachers based on feedback from teachers, log file analyses, and randomized controlled studies.