Stephen Davies, Javier Velez-Morales, & Roger King (2005), "Building the memex sixty years later: trends and directions in personal knowledge bases", Department of Computer Science, University of Colorado at Boulder. The Wikipedia article on personal knowledge bases (PKBs) is basically a summary of the technical report. The report defined personal knowledge base systems, described their benefits, reviewed relevant fields of research, and compared systems in terms of several aspects of their data models: structural framework, knowledge elements, schema, and the role of transclusion. This report is the most comprehensive publication I've read that compares PKB systems according to their key features.