I'm one of the participating authors, and I'll be featuring the first three volumes of The Chaos Chronicles. Yeah, I've had them up for free for two years.
But right now I'm working hard on new afterwords--reflections on the series, and how I went about writing the books. A little added value that I want to add to the books in time for the sale! (But if you buy the books without the afterwords, don't worry. You can just redownload them once the revised versions are up.)