The Future is Unknown — James Valentine

The Adventure Ends. In the third millennium, Jules is suddenly very popular, for reasons he doesn't really understand. He should be happy, but all he can think about is Theo. Are Jules and Gen ever going to see him again? Meanwhile, in the fifteenth billennium, Quincy Carter One has disappeared after the world witnessed his aborted attempt to undermine the entire structure of Time. Not only that, TimeJumping has been all but outlawed on the Two Planets. And Theo can't stop worrying about Jules and Gen -- and what Quincy may be planning, wherever (or whenever) he is. It's the thrilling conclusion of the TimeJumpers trilogy, and no one knows what's going to happen next. Or before. But that's what happens when you start fooling around with time.