Since some people seem to have missed it: the final Discworld book, and Pratchett's deliberate "signing off," is the last Tiffany Aching book (The Shepherd's Crown), not Raising Steam.

I can't really say much about the central theme without giving away a disc-shaking change to the world, because it's about that event and the next generation (Tiffany) carrying on. It's a very sad story, but also meant to be encouraging, and clearly intended to be his last book.