Because he's not the biggest, he's not the bravest, and he's certainly not the most powerful - but he is the boldest, and the brashest, and that makes for a wickedly fun read with Bartimeaus, the fourth level djinni who would much rather be left alone than summoned to do the bidding of foolish human magicians.
The Bartimeaus Trilogy and the fourth book, The Ring of Solomon by Jonathan Stroud, are some of my favorite examples of tone. The humor is pitch black, the voice is absolutely perfect, and the story stays true to it's first person narrator's character, even to the bitter, not-even-for-a-second-saccharine ending. Right at the open, the reader understands that a djinni will turn on a magician in a heartbeat, and not for one second, not even after he's shared three books of adventures with the young magician Nathaniel, do we stop believing that old Bartimaeus wouldn't take that kid out in a second, if he had the chance.
He's a captive, but a wild animal at heart, and Stroud's books never betray that central component of his character.
If you can put up with footnotes (and they're worth it, peeps, really) and you haven't given these books a look, get on it!