The reference parts of the book were already out of date by the time it reached bookstores. (Just the revamped combat log in 2.4 invalidated quite a few pages in the events reference.) As WoW evolves, the API changes too fast for any book to have a complete and current reference to it all. That’s why the book has a companion website — the reference section is kept up-to-date there.

The non-reference stuff will keep its relevance and accuracy much longer, though. True, some of it might be invalidated if Blizzard changes the addon system for Wrath as much as they did for BC… but not much, as most of the core tasks of addon authoring are likely to stay the same. Blizzard would have to massively re-architect its UI system for the instructional portions of the book to completely lose their usefulness — and if they did that, they’d have to rebuild their own UI from the ground up, when I’m sure they’d rather be spending their time building stuff that’s actually new for the expansion. Or to put it another way, we can predict the future by looking at the past: send the book through the Caverns of Time back to the days of WoW 1.5 and a budding addon author would still learn much from it… only a few chapters on advanced topics really involve stuff that’s new since then (or since 2.0).

(No, I don’t receive royalties or commission as a technical editor, I just care about my work! :D )

What I find compelling about the book is the other half. People often ask me how to get into WoW addon authoring; before the book came along, the best I could answer was, “Hang around the WoWInterface discussion boards, browse through the UI/API pages at WoWWiki, and dig into Blizzard’s and other addon authors’ code until hopefully you learn something by osmosis.” There’ve been decent tutorial resources for the Lua language itself and good references to the WoW API out there (and a wonderfully supportive community of addon authors), but the book is really the first thing that brings it all together and tries to teach WoW programming as a whole.