MAJOR SPOILERS ABOUND AND ABOUND ABUNDANTLY. PRETTY MUCH THE ENTIRE REVIEW IS ONE BIG SPOILER SO DON'T READ THIS IF YOU DON'T WANT TO BE SPOILED. 
 The entirety of the plot (if you can call it a plot) in this book could have been avoided had Nikki let her love interest (or as I call him - What's His Face) say "Nikki, I have some esplainin' to do." 
 Or she could have simply asked "Hey dude, why was that girl in your room?" And with that, it is likely that none of the events in this book would even have happened. 
 The fact that she overreacted that massively and hence decided to (view spoiler)[sacrifice herself to the underworld because she saw a girl WALK OUT OF HER BOYFRIEND'S ROOM (hide spoiler)] does not exactly endear me to her as a main character. It doesn't exactly lend her any respect as a strong woman either. 
 I don't really have much else to say. Except.... Ummm.... Mythology?! Yeah there wasn't really much of that either. Eh. *shrugs*