A crucial aspect of the Semantic Web is the capacity to add formalized meanings to information to enable non-human actors to process it. This is usually accomplished by linking the information to an ontology that describes the domain's concepts. In the Web's context it does not seem realistic to represent this semantic layer on a central server, as this model would not reflect the characteristics of presently used, non-centralized networks like the current Web. Therefore we are confronted with a huge number of locally developed and stored ontologies, and we need some kind of integration techniques to connect ontologies developed for the Semantic Web. In this article, we describe our experience with ontology-based e-learning systems and we propose a mechanism to integrate such systems into a Semantic Web context. We concretely present our hyperbook model and show how hyperbooks can be integrated into digital libraries by an ontology mapping procedure.