In order to support various requirements from the user’s perspective, digital library (DL) systems may need to apply a large variety of services, such as query services for a specific DL, mapping services for mapping and integrating heterogeneous metadata records, or query modification and expansion services for retrieving additional relevant documents. This paper focuses on exploiting an extended Service-Oriented Architecture - Peer-based SOA(PSOA) for DL development with the goal of alleviating the weaknesses in the basic SOA infrastructure, especially in the aspects of scalability and interoperability. We also present our work in how to combine the Semantic Web and Web Services together to support interoperability over heterogeneous library services. A query service example is also presented.