*shrugs* It's been speculated in different news and fan sites that Joss released his "ending" first, just in case he can't make three movies. Thus, prequels isn't entirely out of the question.

But in all actuallity, it could just mean flashbacks. Could be that there might have been a side-story arc for Zoe getting over Wash in future installments. Joss Whedon has been known to use flash backs to show deeper meaning or emotion.

Book, on the other hand, would almost definately be used in flashbacks to his past, as his backstory hasn't been revealed yet.