This is a very nice little book, filled with Faerie and language that is exceedingly ornate but never quite pretentious. And it’s a modern fairy tale at that - September, the protagonist, didn’t grow too close to my heart (maybe I’m too old for that), but the narrator did, and maybe that is good enough.
Further Reading
Alice would be the obvious recommendation for a girl dealing with nonsensically fantastic worlds on her own, but I don’t actually think the books share anything except superficial points.
If you like the portal aspect, consider looking into the Wayward Children books. If you liked the heroine most of all, take a look at Coraline, if you dare.