On the Edge by Ilona Andrews
Synopsis from GoodReads:
Rose Drayton lives on the Edge, between the world of the Broken (where people drive cars, shop at Wal-Mart, and magic is a fairy tale) and the Weird (where blueblood aristocrats rule, changelings roam, and the strength of your magic can change your destiny). Only Edgers like Rose can easily travel from one world to the next, but they never truly belong in either.
Rose thought if she practiced her magic, she could build a better life for herself. But things didn’t turn out how she planned, and now she works a minimum wage, 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 her (and her power).
But when a terrible danger invades the Edge from the Weird, a flood of creatures hungry for magic, Declan and Rose must work together to destroy them—or they’ll devour the Edge and everyone in it.