The first line of the book description basically sums this whole thing up:
In his house at R’lyeh, great Cthulhu lies dreaming... of her.
What would you do if you discovered you were the only one in the world with the hidden power to keep it from utter annihilation?
What if you had no idea what that power might even be?
Andromeda Slate, the self-proclaimed most ordinary girl in America, can’t figure out why the gorgeous but mysterious new boy at high school seems to hate her so much. It couldn't have anything to do with the strange dream she had the night before he first showed up in class, could it? The dream where the very same boy rescued her from a giant, green, tentacled sea monster?
And it couldn’t have anything to do with that time she read aloud from that ancient tome of eldritch magic, the Necronomicon... could it?
Andi Slate never imagined she’d find herself in a situation where somehow she was the key to saving the world.
Her life is about to get a whole lot less ordinary.