Macmillan Audio, ISBN 9781250790101, January 2021
Regan is a young girl who has just learned several disturbing things, about herself, and about the untrustworthiness of "friends" who are very, very invested in "normality" and conventionality. She runs away, from school intentionally, and from home accidentally, when she stumbles upon, and then through, a doorway in the woods near her home. The door, of course, should not be there. Neither should the much bigger wood, and wide green fields, that she finds on the other side.
The unicorn she sees is beautiful, but of course impossible, and the centaur pursuing the unicorn even more so.