This is a Wayward Children book, but stands apart from them – none of the action takes place at Eleanor West’s school.
Regan loves horses. She’s got a great relationship with her parents, but a rather shocking secret challenges her relationships with the girls she thought were her friends. (The set up for that is actually a really interesting case study of girls’ cliques in school.) So her trip through a door lands her in the Hooflands, where everyone has hoofs – so you’ve got unicorns, and centaurs, and a hosts of other hooved beings.
Regan falls in with a clan of centaurs, and finds friendship there. The complication is that any human found in the Hooflands needs to be taken to the Queen, but she and her friends put that off as long as possible.
I accidently ready the last book first, so I know what happens after she accidently steps in the door to return to her home – kinda glad I read that first.