There are a ton of children's fantasy novels which involve a girl, or a boy, or sometimes a group of them wandering off from our reality into some other reality. Sometimes the access is through a wardrobe, or going out in the foggy twilight. Sometimes it's going into a shop and coming out into another world.
Our Heroine, Claire, grows up in the Mudflats, a part of Seattle occupied by other misfits and somewhat magical people. Her parents took off separately, leaving her with aunts and ultimately with her grandmother whose house she inherited after taking care of her for a little while. A troll lives in the basement.
Claire makes some bad dating decisions and needs to leave town in a hurry to avoid the most recent suitor more or less kidnapping her to force her to use her magical skills (her horoscopes are accurate and relatively detailed) to assist in his criminal enterprises. She tags along with friends camping in the Olympics. Once there, they do drugs and get a little weird and she decides to hitch her way somewhere else, but takes a short cut out of camp and gets really, really, really lost.
Once lost, Our Barbarian Hero (sort of), Tarvik, kidnaps her. Other hikers have been lost this way before and the last batch through saved Tarvik's dad's life. They have since died themselves, but Tarvik's dad continues to worship them, sort of. Tarvik sees a family resemblance and brings her back to be a priestess in their temple. The local warlord setup is kind of a crap deal, even compared to camping while looking over one's shoulder, but it takes a few months before Claire is able to make a break for it in good conscience. The intervening adventures make up the bulk of the book.
Claire does successfully figure out (more or less) the connection between this world and ours, and come up with a way to find the door or hole or whatever she slipped through and retrace her steps to get back home. The epilogue sets up book 2: Claire brings the local priestess with her. And Our Enterprising Barbarian Hero, Tarvik, sneaks through as well.
I don't know that I've ever read an adult version of the slip-through-the-door-to-another-world fantasy. There's no sex in this, so it's not _that_ adult. It works a lot better than I expected it to, altho it doesn't hurt that the setting is both familiar and beloved to me.
ETA: Looks like a five book series (to date) with short stories in the universe. I'll continue.