Shadows of the Past - Book 1 in the What Isn't There series

Christina July is your average teenager. She has a best friend she's know since grade school, a truly inspiring level of hatred for high school, and more than a few issues with being told what to do. She's also been hallucinating the same little boy since she was a child, although lately they seem to be turning into something more than just her imagination. So when a car crash accidentally launches her through a rift in space and time, she's forced to confront the side of her family she's never known and been to afraid to ask about.
Along the way she befriends Nico di Angelo, a moody demigod who likes to seem tougher than he is, Percy Jackson, her older half-brother and apparent savior of the world twice now, and Leo Valdez, a boy she not only rapidly finds herself intrigued by but also the most enticing reason she has to stay.
Can Christina not only navigate boy drama and the chemistry homework due tomorrow, but also monsters and the apparent fate of the world as they know it on her shoulders? Maybe, if her calculus class doesn't kill her first.