On the Edge by Ilona Andrews
The Edge #1
Rose Drayton lives on the Edge, between two worlds: on one side lies the Broken, a place where people shop at Wal-Mart and magic is nothing more than a fairy tale; on the other is the Weird, a realm where blueblood aristocrats rule and the strength of your magic can change your destiny. Only Edgers like Rose can easily travel between the worlds -- but they never truly belong in either.
Rose thought that if she practiced her magic, she could build a better life for herself. But things didn't turn out the way she'd planned. and now she works an off-the-books job in the Broken just to survive. Then Declan Camarine, a blueblood noble straight out of the deepest part of the Weird, comes into her life determined to have Rose (and her power).
But when a terrible danger invades the Edge -- a flood of creatures hungry for magic -- Declan and Rose must overcome their differences and work together to destroy them, or the beasts will devour the Edge and everyone in it...