Nathaniel: I tried last night and you were gone. Who was it? Which magician were you seeing?
Bartimaeus: Donít get so worked up. It was a brief encounter. Nothing serious. Itís over.
Nathaniel: Nothing serious? Think Iím going to believe that?
Bartimaeus: Calm down, Mr. Jealous. Youíre making a scene.
Nathaniel: Who was it? Man or woman?
Bartimaeus: Look, I know what youíre thinking, and I didnít.
Badass Decay: Bartimaeus throughout the trilogy. Justified in that he has weakened severely due to having stayed in the mortal world for far too long, to the point where he starts to literally decay due to his failing essence.
Designated Hero: A good deal of the series is dedicated to subverting, lampshading, playing with, and examining this trope.
Why do we not see any technology more advanced than the telephone? Because with Magicians using magic and demons for things like sending messages, spying and in general carrying out their dirty work there was no demand to develop more advanced technology. The Magicians certainly don't want the Commoners to have access to anything that could threaten them or their power, so don't let things like walkie talkies, advanced bombs or mobile phones be invented.
Commoners aren't even seen with conventional weapons like guns. Think of how a metal bullet would affect a demon, considering it's their main weakness.
Magicians aren't allowed to have children. The reason stated is because family feuds inevitably happen. However if Commoners are able to develop resilience just by the presence of demons in the city, then imagine what might happen if an unborn child has heavier exposure to magic.
Foe Yay: Nathaniel and Kitty in Book 2. Ptolemy's Gate makes it more obvious; Bartimaeus is most amused by it.
"Funny Aneurysm" Moment: Bartimaeus' aside halfway through book 3 about the impracticality of lions' manes in combat seems like just another of his regular doses of snark at the time. Cue the final Ptolemy chapter, and we discover he was speaking from rather tragic experience.
After having seen another insane, skeletal villain rant about the advantages of having a body, many readers will be tempted to read Honorious's lines with the voice of James Spader in Avengers: Age of Ultron.
Ho Yay: Uh. Ptolemy, the few times he and Bartimaeus interact. Can also be seen as Intergenerational Friendship - after all, it's YMMV. At least, the subtext is there to be seen that way.
With Nathaniel, too. For all his complaints about Bartimaeus, Nathaniel keeps finding excuses to bring him back, even when he could have used a more powerful or efficient servant. Bartimaeus is also the closest thing Nathaniel has to a friend and is the reason Nathaniel didn't become like the other magicians.
Khaba and his marid Ammet in the prequel. Good grief.
Faquarl/Bartimaeus is a surprisingly popular slash pairing in the fandom.
Lovelace is clearly a scumbag from moment one, but he really crosses the line when he orders Jabor to kill everyone in Nathaniel's house (including the innocent Mrs. Underwood) even though Nathaniel confessed that only he was involved.
Arthur Underwood crosses it when he's perfectly willing to let Lovelace kill Nathaniel to save his own skin. He may have crossed it even earlier when he catches Nathaniel with summoning equipment, and suggests that he's thinking of killing him on the spot, and only doesn't do so because "forms would have to be filled out" and "his judgment would be called into question."
The description of just what exactly is in Khaba's vaults. Mummies, multiple skeletons (of both animals and humans), tortured spirits...
Gezeri's final attack on Asmira - he's mentioned as having lots of sharp teeth and claws, as well as skittering with "horrible speed" like some sort of demonic spider.
The most powerful spirits are enslaved for thousands of years and if lucky get only a few years in-between periods of captivity. Imagine being cursed with a potent combination of raw physical strength, cunning and intelligence and being forced to serve as a slave for thousands of years.
Paranoia Fuel: In one of the footnotes in which Bartimaeus is describing the nature of the demons, (specifically the fact that they can't be seen unless you have special glasses or they want you to see them) he remarks that there is probably something with lots of tentacles sitting behind you right... about... NOW!
Kavka in Golem, and a good chunk of Gate, particularly Nathaniel's meeting with Ms. Lutyens.
Really Bartimaeus' entire relationship with Ptolemy.
"It's two thousand, one hundred and twenty-nine years since Ptolemy died. He was fourteen. Eight world empires have risen up and fallen away since that day, and I still carry his face. Who do you think's the lucky one?"
The death of Nathaniel at the end.
Kitty: [speaking at the ruins where Nathaniel died] So much for your promises.