Besides the thesaurus book below, we need a book that addresses metadata design from the LIS, IT, UCD and knowledge representation points of view together: the IA metadata book. I think LIS knowledge is well-established, UCD and usability practice have evolved pretty well in this respect, and knowledge representation is being wrenched from AI circles into practical use, but I don’t see them brought together in a useful way yet.
For example, I’ve realized that there are multiple ways to model the same information, e.g. more or less faceted vs. hierarchical, or level of richness of semantic relationships. The kind of issues that need to be addressed (and I’m just jotting these down quickly for myself you understand) are how to distribute attributes across a hierarchy in a way that both makes for an efficient admin UI and efficient use of inheritance in the system while serving the presentation layer. Also, basic ROI guidelines that guide the sophistication of the modeling, such as when it’s worth the extra work of an ontology’s features vs. accomplishing similar results in simpler ways in the presentation layer using another modeling method.