Lately I've been fixated on second books. Not my own second book (as if!), and not the specific second books of specific people, but rather the idea of the second book: that thing one writes totally on one's own, more or less because one can or wishes to--without the guidance of a dissertation director or committee, and not because one needs it for a job or for tenure. And in thinking about the shape of my field and the players in it, I often wonder: whose second book will be better than his first? And whose won't?
I don't wonder this very deeply about specific people, because the point of the question is that you can't know. When you're a grad student, a junior faculty member, or probably even a mid-career faculty member, there's no way to predict the course that someone else's intellectual development will take over time. Some people are very quick out of the gate, and though a few continue at that speed, many don't. Others start slowly and unpromisingly but then catch fire. (And I imagine that still other people are quick starters who stall out for a while and then speed up again.) Some people's brilliant first books might owe too much to their advisors, or simply the fear and inspiration of the job market. Other people's lackluster first books might be the result of already having moved on to the next book, and just pushing this one out for tenure.
So you know, though I'm as quick to judge as anybody--as prone to say "that article sucked! God, he's a moron!" or "this is the new star of our profession! I shall admire and worship her forever!"--I sometimes mutter "second book" to myself, as a reminder that people are surprising, and that, as the investment-market warning goes, past performance is no guarantee of future results.