The Change series is lots of fun, if you like post-apocalyptic fiction where technology above a certain level abruptly no longer works and the return of magic really doesn’t make up for it at all. Which, yeah, led to billions of people dying. But the survivors are having plenty of adventures! …Which, as S.M. Stirling himself noted elsewhere, means ‘somebody else in deep trouble, far away.’
Ahem. The Desert and the Blade: A Novel of the Change (Change Series) is a fun read so far; I’m enjoying it. But you definitely should start with Dies the Fire, which is the first book in this series. Or maybe Island in the Sea of Time, which is kind of in the same universe but is a good deal more chipper. Assuming you haven’t read any of these before