Polzin Kiera Polzin 
Hematite Horses [EPUB ebook] 

If there is a defining moment where it feels like you are forced to grow up overnight, for Kate Williston it was the day her best friend, Jason, leads her through a Postern door to another world. Kate, who loves to sneak out of the house to see Jason in the middle of the night, gets more than she bargains for when Jason makes the decision to bring her through the door. She is forced to confront the disconnect she feels from her family in a real way when they return and what was just a day for them, turns out to be fourteen months back on Earth. In the end, Kate must convince Jason that what was broken is not in her power to put right, and he should let her go with him again, this time maybe for good.