Am 10.01.2015 um 15:18 schrieb Gerben Wierda :

I have a chapter in the frontmatter that has no chapter number. The first chapter in the real book is Chapter 1.

But when I refer to \in{chapter}[prologue], what I get is "chapter 1". Is there a way I can make \in{chapter}[prologue] return something like "the prologue“?

You can use \about[prologue] to print the title of the heading. Wolfgang