This is a strange and imaginative book about an Earth woman from London who is sort-of abducted by cousins from another world. She is taken to a fairyland full of satyrs and giants and faerie-like humans, and finds herself caught in the middle of a war between those who live inside the magical mountain that is the source of the faerie beings' immortality, and those who have been exiled to the outside.
This book was written by the author when she was fourteen. They decided to publish it on the strength of her storytelling and her imagination; it wasn't on the strength of her writing, which is quite immature and often just plain ungrammatical. It takes a while for the story to go anywhere and there is a lot of irrelevant description, but there is an imaginative story and some sparkle of brilliance buried beneath the adolescent purple prose.
I want to give it a higher rating, taking into account the fact that it was written by a fourteen-year-old, but objectively, it's not a great fantasy novel and not really that original, and it's marred by the poor writing. If I were judging it relative to, say, NaNoWriMo novels and other output from teenagers and amateurs, I'd give it a higher rating, but compared with professional published fiction, it just doesn't have that much to commend it aside from the novelty of having been written by a kid.