Felicia is dead. And she's stuck in Level 2, a limbo between Earth and your final resting place. And boy, is Level 2 boring. (The limbo, not the book, I mean.) Its purpose is to revisit your memories and move on, but something's a little wonky--no one's moving on. They're watching their happy memories over and over, perfectly content dwelling on their former lives. 
 And then comes Julian. Felicia doesn't have fond memories of Julian from Earth, but he's blabbering about a rebellion of some sort, and she suspects that there's must something more than just... this. So she escapes with him. 
 I love novels that explore the unknown, and nothing is more unknown that death. It uses tales from mythology, like the rivers of hell, and religious concepts. Not all angels are good, etc. I'm pleased to see someone discussing the fallen angels for a change, and showing that they're not all perfect, heavenly beings. 
 I'm a little torn on the execution, though. Felicia is pining over her boyfriend, Neil, and Julian convinces her to join him so they can be reunited. Yeah, when you're seventeen years old your boyfriend is the most important person in the world. But as a major plot point? Neil isn't that interesting. He keeps Felicia going, but her entire [after]life is focused around someone who's not all that essential to the plot at all. 
 But I like the way this is laid out. As Felicia is learning of Julian's rebellion, she's also coming to grips with the not-so-happy parts of her life. She plugs into her memories and we view them in flashbacks, which ultimately help form her character. (The Bad Thing that happened to her could have been developed more, but I'm just being picky.) 
 I'm super-confused by the ending, though. Maybe it's explained more in the sequel. And how does Felicia actually aid in this rebellion, as we're taught to believe throughout the entire thing?