Sword Catcher by Cassandra Clare

In the vibrant city-state of Castellane, a young orphan named Kel is stolen from his old life to enter a new one of luxury and peril. He’s to become Prince Conor Aurelian’s body-double, shielding the Prince from all dangers. As his ‘Sword Catcher,’ he and Conor become close as brothers – yet Kel lives for one purpose: to die for Conor.
Lin Caster is an Ashkar physician, part of a community ostracised for its rare magical abilities. But events pull her and Kel together and into the web of the mysterious Ragpicker King who rules Castellane’s criminal underworld.

Together, they’ll discover an extraordinary conspiracy. But can forbidden love bring down a kingdom? And will their discoveries plunge their nation into war and the world into chaos?