Well, let's see, in the very first chapter of the book, Zoey is Marked by the vampyre Tracker, which instantly turns her life upside down. So it seems fitting for a title, right?
We think there's a little more to delve into, though. Yes, being Marked changes Zoey's life forever; yes, it gets her out of the bad situation at home; yes, it catapults her into a new life. But getting Marked also has some special meanings in Zoey's life, such as:
The goddess Nyx appears to Zoey in a vision soon after she's Marked, explaining that she has special plans for Zoey.
Nyx also fills in Zoey's outline of a crescent moon (her Mark), causing it to be entirely filled in, which apparently never has happened to a fledgling vampyre before. So her Mark helps set Zoey apart from all the other fledglings.
And by the end of the book, Zoey's Mark is added to when she dispels some hungry ghosts and receives the characteristic tattooing that all mature vampyres get.
In other words, lots of things pertaining to Zoey's Mark make her unique, pointing her toward a greater role to play in the House of Night, and possibly the whole world. Yeah, we think it's fitting for the book's title to hint at all that stuff.