McGuire, Seanan. Wayward children.

"A Tom Doherty Associates book."

"Regan loves, and is loved, though her school-friend situation has become complicated, of late. When she suddenly finds herself thrust through a doorway that asks her to "Be Sure" before swallowing her whole, Regan must learn to live in a world filled with centaurs, kelpies, and other magical equines--a world that expects its human visitors to step up and be heroes. But after embracing her time with the herd, Regan discovers that not all forms of heroism are equal, and not all quests are as they seem... A standalone Wayward Children story containing all-new characters, and a great jumping-on point for new readers"--