The City on the Other Side - Mairghread Scott
Sheltered within her high-society world, Isabel plays the part of a perfectly proper little girl—she’s quiet, well-behaved, and she keeps her dresses spotlessly clean. She’s certainly not the kind of girl who goes on adventures.
But that all changes when Isabel breaches an invisible barrier and steps into another world. She discovers a city not unlike her own, but magical and dangerous. Here, war rages between the fairies of the Seelie and Unseelie Courts. Only Isabel, with the help of a magical necklace and a few new friends, stands a chance of ending the war before it destroys the fairy world, and her own.