The Keeper (Book 2: The Guardian Series):

Clare has woken up in the desert with no memory of her past and no true direction, beyond the impulse to walk west. She has never been more lost. To add to her a problems, an angel keeps trying to kill her, a Watcher wants to control her, and her only clue to understanding the future is to find a city that does not exist. The war is coming, but will she be ready? 'The Keeper' is the second book of The Guardian Series.