In An Absent Dream by Seanan McGuire

Every young person feels lost or insignificant at some point or another and that is why the doorway appears. For Katherine Lundy, never Kate, it offers a much-needed escape, friendship and order to the Universe that makes sense. This wonderful, magical world doesn't treat children as empty-headed nothings but instead offers them choices as long as they give back fair value. It does have some harsh consequences if you don't take care of your debts and some, like her friend Moon, may need more time to grasp the concept but Lundy embraces it and this world greedily. Without thought to the family that is left wondering where and for how long their child will be gone, Katherine travels back and forth. The only thing the door asks is if you are sure. Interesting, unique and thought-provoking, this slim volume is a fairy tale about the magical world children create to give themselves power and control. I will keep looking for those doorways, I'm sure.