The amulet of Samarkand

Nathaniel, a twelve year old magician in training, thinks he's ready to take on more challenging spells. With revenge against the proud and ambitious Simon Lovelace on his mind, he masters one of the toughest spells of all and summons Bartimaeus, a 5,000 year old djinni, to assist him.
But summoning a djinni and controlling him are two different things entirely. When Nathaniel sends Bartimaeus to steal Lovelace's greatest treasure, the Amulet of Samarkand, he finds himself caught in a whirlwind of espionage, murder, and rebellion.