Recently out from Chandos Publishing is Finding the Concept, Not Just the Word: A Librarian's Guide to Ontologies and Semantics, an in-depth look at the theory and application of ontologies by Brandy King and Kathy Reinold.
My research at MINDSWAP building a Semantic Web terrorism research portal was one of their case studies.
The concept, in a nutshell, is that with the enormous amounts of electronic data now available, new tools are needed to search and organize it efficiently. Ontologies help the computer place information in context and allow it search more efficiently. Some ontologies can allow the computer to infer relationships and ease the use of applications such as network graphs and timelines.
An example of some of this work and a description can be found here.