"The column hung above the middle of the pentacle, bubbling ever upward against the ceiling like the cloud of an erupting volcano. There was a barely perceptible pause. Then two yellow staring eyes materialized in the heart of the smoke.
Hey, it was his first time. I wanted to scare him. And it did, too."
- Bartimaeus of Ur, his introduction. The first few lines of the first book.
The Bartimaeus "Trilogy" is a series of books by Jonathan Stroud. The series consists of the original trilogy -The Amulet of Samarkand, The Golem's Eye, and Ptolemy's Gate- and a prequel book entitled The Ring of Solomon. The main three in the series are set in twentieth century London while the prequel is set in (mostly) Jerusalem during the time of king Solomon. Each of the books are told half from Bartimaeus's point of view, and half in the limited-third person from Nathaniel or Kitty's point of view.
Bartimaeus is a several thousand year old, wise-cracking djinni (not demon) in an alternate universe where djinn are forcibly pulled from their miniverse and enslaved to human magicians. The year is some vague time in the 20th century, but thanks to the alternate history, the fact that the availability of technology varies among the classes of people and technological advancement as a whole is pretty sketchy, and the frickin' magic, it feels like it's earlier than that.
In this world, Gladstone was a magician who overthrew the non-magical British monarchy, became Prime Minister, set about conquering most of Europe, and then set up a police state and the modern oligarchy where magicians rule from the top. The big huge secret that magicians are keeping from the rest of the populace? Yeah, magicians actually have no inherent magical superiority. The only thing they have going for them is the ability to summon djinn, an ability that anyone can learn how to do. The colonies in this 'verse are only just starting their revolutionary war (and several times throughout the series, the threat of being sent to the Americas is used on the djinn to keep them in line, as apparently this version of the war is a lot more bloody and a lot more magical in nature).
The first book, The Amulet of Samarkand, follows the story of twelve year old apprentice magician Nathaniel as he summons Bartimaeus and takes revenge on the magician Simon Lovelace and winds up uncovering a plot to kill off a good chunk of the government. This book is set half and half between Nat's POV and Bartimaeus's POV.
The second book, The Golem's Eye, takes place a couple years after the first, with Nathaniel, now working in the government under the Minister of Internal Affairs and apprentice to the Minister of Security, on the hunt for The Resistance: a shady group of rebelling commoners who seem to be immune to some magic and can see through illusions. Meanwhile, a giant thing (three guesses as to what it is, and the first two don't count) has been breaking into magicians shops and Nathaniel once more has to summon Bartimaeus and get his help in dealing with it. Meanwhile, Kitty Jones, a member of the Resistance and born with resilience- an immunity to magical attacks- and her crew are encouraged to break into Gladstone's tomb in search of a powerful magical artifact by a mysterious benefactor. . .
The third book and end of the original trilogy, Ptolemy's Gate, takes place three years after The Golem's Eye. Nathaniel is now a big hot shot in the government and the sharks are out for blood. Foreign wars are kicking up and going badly, commoners are starting to chafe with the government, and the natural resilience to magic being born into kids is finally setting in, with imps and djinn being accosted by kids in the street, and everyone in the government is either wanting Nathaniel to fix it, or will blame him for it if it doesn't get better. All the while, Bartimaeus has been stuck in the physical world for far too long because Nat won't let him leave. This is of course the perfect time for Nathaniel to have a mental breakdown, identity crisis, and massive guilt trip. Kitty Jones has been living a quiet life helping a magician who is actually not an asshole and spends all her time trying to understand the nature of the djinn, to the point where she learns how to summon them, just so she can talk to Bartimaeus again.
Meanwhile, a group of djinn are starting to act awfully suspicious, and it becomes clear that the commoners aren't the only ones thinking about revolution. . .
The fourth book and prequel to the others, The Ring of Solomon, takes place during, big surprise, the time of king Solomon. Bartimaeus is forced to serve under a particularly cruel master for his shenaniganry (killing and eating his old master), saves the life of a female assassin named Asmira sent by the Queen of Sheba to Solomon, and winds up getting tangled in not one, but two different plots to kill Solomon.
The books are brilliantly written and funny as hell. Go read them.