These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
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.
Foe Yay - Nathaniel and Kitty in Book 2. Ptolemy's Gate makes it more obvious; Bartimaeus is most amused by it.
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.
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?"