Inside the Illusion [Everworld #9] by Katherine Applegate

Senna. She is the reason David, Christopher, April, and Jalil even know about Everworld. She is the one who they've been chasing all over this strange parallel world to get some answers. To show them how to get home again. To get back to the real world.
So now it's her turn to tell her side of the story. To explain why she dragged her friends into a world of winged horses and wizards. A place that once only existed in their imaginations. Now it's her turn to explain Everworld...