This is one of my all time favorite trilogies EVER! The Infernal Devices is a series of novels by author Cassandra Clare, centering on a race called the Shadowhunters introduced in her The Mortal Instruments series. The series is a prequel series only in that it takes place in an earlier time period to the Mortal Instruments series, and contains some of the character's ancestors.
The series follows Tessa Gray, an orphaned teenage girl who discovers she has the power to shapeshift, but doesn't bear a mark that shows she is a warlock, and abilities that plunge her into a world she never knew existed. She will have to learn to master them if she wants to find her brother Nate and she will have to forge an alliance with Shadowhunters if she wants to survive in this dangerous world.
The trilogy features Will Herondale, Jem Carstairs, Tessa Gray, and other Downworlders and Shadowhunters in the London Institute. And who could not absolutely fall in love with Will? Well if you are a guy than maybe not. ( I bet you would be jealous though.)