Ten Thousand Skies Above You By Claudia Gray

Ever since she used the Firebird, her parents’ invention, to cross into alternate dimensions, Marguerite has caught the attention of enemies who will do anything to force her into helping them dominate the multiverse—even hurting the people she loves. She resists until her boyfriend, Paul, is attacked and his consciousness scattered across multiple dimensions.
Marguerite has no choice but to search for each splinter of Paul’s soul. The hunt sends her racing through a war-torn San Francisco, the criminal underworld of New York City, and a glittering Paris where another Marguerite hides a shocking secret. Each world brings Marguerite one step closer to rescuing Paul. But with each trial she faces, she begins to question the destiny she thought they shared.