This book was creepy, and it was weird, and it was fantastic.
I was drawn in mostly by the title and the concept--I'm a sucker for the "normal person in a fucked up magical family" trope--and by the end I was blown away. It's a slow-burn creature horror with a weird concept that just works. The characters are all really interesting, dimensional, imperfect, and well-written, with complex and fucked up backstories that help tie everything together. There's a darkness, a ferality to the writing style and the main character's narrative voice that gives a great edge to the story. It's a little slow-paced towards the beginning, but it doesn't drag; it's a great slow-burn that helps create tension that builds and builds as the story progresses, and it eventually does pick up in pace (I'm not usually a fan of slow-burn, but it's just done so well here). 
My only note is that it's not explicitly stated when the story takes place; you can garner easily enough that it's not set contemporarily, between the way they talk about certain things and the absence of cell phones, but it's not clear exactly where the story is temporally. I don't mind it, actually; I think the vagueness contributes nicely to the tone of the story, but I can understand how some might find it irritating.
What Big Teeth is such a great debut, and I can't wait to read more from Rose Szabo. Fully recommend this for anyone looking for something spooky, unsettling, and just a little off-kilter.