City of Fallen Angels by Cassandra Clare

Cassandra Clare's City of Fallen Angels, the fourth book in The Mortal Instruments series, explores the fragile peace between Shadowhunters and Downworlders which is threatened by mysterious murders. Simon Lewis, now a Daylighter vampire, finds himself central to these events and his complex relationships with Isabelle Lightwood and Maia Roberts are further complicated. Meanwhile, Clary Fray and Jace Herondale's intense bond faces new challenges, including Jace's disturbing nightmares and the manipulative influence of a powerful demon, Lilith, who seeks to resurrect Sebastian Verlac. The narrative follows these intertwined storylines as the characters confront betrayal, make difficult choices, and battle supernatural forces in an effort to protect those they love and maintain order in their world.