Chain of Gold by Cassandra Clare (Book 1:The Last Hours trilogy)

Cordelia Carstairs arrives in London hoping to restore her family’s reputation. She reunites with childhood friends James and Lucie Herondale, and is quickly swept into their glamorous and secretive world. But London is suddenly plagued by a new kind of demon attack—one that leaves its victims paralyzed by daylight and immune to typical weapons.

As Cordelia and her friends—including the charming Matthew Fairchild and bold Anna Lightwood—investigate the mystery, secrets from the past come to light. Meanwhile, Cordelia finds herself falling in love with James, who may be bound to someone else by magic and fate.