This paper describes the electronic ACT-R Bookshelf, a system which supports learning ACT-R, a well-known theory in the field of cognitive psychology, over the World Wide Web. ACT-R Bookshelf is a collection of electronic books on various aspects of ACT-R. The primary role of ACT-R Bookshelf is to serve as a 24-hour information resource for Carnegie Mellon University (Pennsylvania) on-campus students taking a specific course; however, the system can be used as a resource for distance learning of ACT-R, as well as an information resource for the international community of cognitive scientists and ACT-R researchers. The books on ACT-R Bookshelf are authored and served on the Web with the InterBook system. This paper discusses concept-based knowledge representation behind adaptive electronic textbooks on the bookshelf, describes the main functionality of the system, provides some evaluation data, and speculates about prospects for ACT-R Bookshelf as an integrated resource on the Web. Two figures present a section of ACT-R course material, one with a quiz and the other with a glossary window. (Author/AEF)