The Amulet of Samarkand by Jonathan Stroud

Nathaniel, a magician's apprentice, has learned a spell to unleash a 5,000 year old djinni, Bartimaeus. Nathaniel wants Bartimaeus to steal The Amulet of Smarkand from a very powerful magician, Simon Lovelace. Nathaniel is angry at Simon and wants to get even with him. Bartimaeus does indeed steal the Amulet and when Simon finds out where it is, he kills Nathaniel's master, Mr. Underwood. Nathaniel again decides to get even and get the amulet back. When he attempts to trap Simon and retrieve the necklace, he unleashes an evil power that all of the magicians in London cannot stop.