No one writes about discourse analysis as expertly and engagingly as Gee. His approach is complicated (it has to be, as language use itself is complicated), but his explications and examples are, though sometimes redundant, always appropriate and accessible.
Two minor gripes: Gee recycles data and its accompanying analysis from his other books and articles frequently. While it means you may have already been exposed to these analyses elsewhere, your ability to apply particular tools for particular situations is enhanced, as the sociological significance of a particular data narrative takes a back seat to the practice of CDA application. Lastly, for a book that focuses so specifically on careful attention to language construction, and this being a fourth edition, where the hell was the editor? Dropped pronouns, articles, even some dropped nouns abound. Routledge, one of the most distinguished academic publishers, should be ashamed.