If you do any InDesign scripting (or want to learn), or create any InDesign GREP searches (or want to learn), you need these eBooks by Peter Kahrel. If it were possible for eBooks to become "dog-eared", mine would be. I refer to one or both of these books almost every week.
GREP in InDesign CS3/CS4 is a 65-page tutorial and reference guide for how to do complex, powerful searches in InDesign using GREP.
Despite the titles of these guides, Peter has recently updated both of them to cover InDesign CS5, as well as earlier versions. These are the only two published guides that I know of that cover these two subjects...I'm glad that Peter has done such a great job.