This paper describes a digital library architecture and implementation that is configurable, extensible and dynamic in the way it presents content and in the services it provides. The design manifests itself as a network of modules that communicate in terms of XML messages. All modules characterize the functionality they implement in response to a describe yourself message, and can transform messages using XSLT to support different levels of configurability. Traditional library values such as backwards compatibility and multiplatform operation are combined with the ability to add new collections and services adaptively. The paper describes the new design and shows how it can be used to build four different digital library systems. We conclude by showing how the design fits existing interoperability frameworks.