The Bartimaeus Sequence[1] is a series of children's novels of alternate history, fantasy and magic. It was written by Jonathan Stroud and consists of a trilogy published from 2003 to 2005 and a prequel novel published in 2010. The eponymous character, Bartimaeus, is a five-thousand-year-old djinni, a spirit of approximately mid-level power. The story follows the career of a teenage magician Nathaniel and Bartimaeus, whom he has summoned and nominally controls through the alternative history of the peak of London's domination as a magical oligarchy.
https://en.wikipedia.org/wiki/Bartimaeus_Sequence