After the improvement and the greater scope of Magician: Master, Silverthorn marks a rather brutal return to immaturity in pretty much every aspect and, for the most part, a simple heroic fantasy adventure. It's quite forced as a sequel at first, but eventually it finds its way and elements start to connect, albeit not necessarily in a smooth or, at times, even reasonable fashion, so it eventually grows and opens the way to another bigger story, but it does not contain it.
Expectations are very different today, or at least mine are, but, keeping them in check, I wasn't exactly bothered by anything while reading it and it actually went faster and smoother than I expected. Taken for what it is, assuming teenage boys as the target audience once again, recognizing the fact that the fantasy genre has improved massively in the decades since it was written, and not expecting details that aren't absolutely necessary to the action, carefully developed and developing characters, or significant depth or wisdom, there's not much that can be said to be particularly wrong with it. That's just far from being enough, however.