The infernal devices clock work angel (book one) - Cassandra Clare

Magic is dangerous -

but love is more dangerous still

When sixteen-year-old Tessa Gray arrives in England during the reign of Oucen Victoria, something terrifying is waiting for her in London's Downworld, where vampires, warlocks and other supernatural folk stalk the gaslit streets. Friendless and hunted, Tessa seeks refuge with the Shadowhunters, a band of warriors dedicated to ridding the world of demons. Drawn ever deeper into their world, she finds herself fascinated by - and torn between - two best friends and quickly realizes that love may be the most dangerous magic of all.