The Software Engineering Institute (SEI) of the Carnegie Mellon University has a very interesting report, which I recommend if you are interesting in e-Government, specially related to interoperability field. The report is addressed to understand correctly e-Government interoperability concept. This SEI document explains what is e-Government, and what are the e-Government interoperability problems, proposing a model for understanding interoperability in the e-Government context. The report also presents a guidance on how to address interoperability requirements and describes challenges. Don’t miss it; it is really useful. You can download here the full document.