Essentially, this book is a coming of age story. With magic (set in the late 1970s, in Wales and England). The voice of the protagonist is achingly real- the entire story is her diary entries, so we see the world through her eyes. She doesn't want to think or talk about her mother (or her father, much), nor does she go into what happened to her twin sister.
In fact, for the first half of the book I was thinking she was an unreliable narrator (maybe insane, as she claims her mother is) and by the end of the book I was *still* wondering how much of this stuff was in her head. Real for her, but not for the rest of us. That's the magic of this book- the system of magic is "always deniable", and makes a beautiful sort of rule-less sense, so of course the entire story is also deniable. And breaks conventional rules of storytelling.
If you go into this novel expecting an epic showdown, as the synopsis hints, you will be disappointed. The pace is steady but slow, with little to no rising action until the climax itself (in the last 10 pages of the book). But the language, and the process of Mor essentially crawling into the world (not giving up her books, but not hiding behind them of her past anymore), is the true story.
I was surprised this won a Hugo, because the narrator's primary positive experiences with the world are with fairies, which always seem a fantasy element to me. But Mor is an avid reader, and loves science fiction (and given the setting, her SciFi books are the classics) and talks at length about the brilliance or insipidness of the books she's reading. So in a way, it felt like this real person (or perhaps the author? It does feel somewhat autobiographical) was recommending books directly to me.
If you like thoughtful, unique novels that aren't necessarily an adventure story, classic science fiction, magical realism, contemporary coming of age (where love isn't the trite answer to everything), a potentially unreliable narrator, and an authentic MC voice, I highly recommend this one.