City Of Glass by Cassandra Clare
In this urban fantasy novel, the third installment of a series, the protagonist, a young Shadowhunter, travels to the titular city, a place of great significance to the secretive world of demon hunters, warlocks, vampires, and other supernatural beings. She is on a quest to find a cure for her poisoned mother, but her mission quickly becomes entangled with a larger conflict as she uncovers dark secrets about her family's past and the true nature of the Shadowhunters. As the city becomes a battleground, alliances are tested, and the protagonist must confront a powerful enemy who threatens to destroy everything she holds dear, all while grappling with her evolving feelings for her enigmatic companion.