The Bartimaeus Trilogy/WMG

The Bartimaeus Trilogy is set in the same universe as Shakespeare's The Tempest.
"Ariel: Your charm so strongly works 'em
 * It scares me that this makes so much sense.
 * Less mysterious if you figure that Jonathan Stroud consciously drew on Shakespeare for inspiration.
 * YES!!! This troper was discussing The Tempest in her Shakespeare class the other day and came to the sudden realization that the relationship between Prospero and Ariel almost exactly mirrors the one between Nathaniel and Bartimaeus, right down to the threat of punishment by confinement in a small prison (tree knot instead of a tin at the bottom of the Thames) if Ariel doesn't carry out orders. There's also the extension of Ariel's contract without his consent, the substantial question of whether he and Prospero have any goodwill for each other, and the fact that it is eventually Ariel that brings Prospero's attention to the lack of humanity in his treatment of the other men. There's even this quote, which, modernized slightly, would fit perfectly into any of the Bartimaeus books:

That if you now beheld them, your affections

Would become tender.

Prospero: Dost thou think so, spirit?

Ariel: Mine would, sir, were I human.

Prospero: And mine shall."


 * In the same vein, Alexander Pope's The Rape of the Lock fits nicely into the same universe: it features an Ariel (who may or may not be the character from The Tempest) once again leading a team of lesser spirits to wait upon their mistress (albeit doing much more menial tasks: instead of foiling assassination attempts and bringing usurpers to justice, he is babysitting a lapdog, helping his mistress cheat at cards, and shielding her expensive brocade gown from spilled coffee).
 * If you accept the shared universe theory, it makes a nice chronological bridge between the era of Prospero (and Shakespeare) and the decadence of John Mandrake's London. Seriously, Belinda (real name: Arabella Fermor) employs a hundred imps just to keep her petticoats fluffy during a lengthy soiree (and Ariel warns that an attendant caught shirking his task will have his "essence" boiled in a chocolate mill). And if you thought John Mandrake was prissy about his hair...

All the demons are souls of the dead summoned for those who wish to use their new abilities.
How would people know the names of demons unless they started guessing?
 * Word of God says demons are unnamed before they are first summoned and given a name - they're not even individuals before being ripped away from the Other Place the first time. Though Jonathan Stroud has expressed interest in the idea that ...
 * Surely Bart isn't that cruel.
 * If any such thing happened (face it, ), it would probably be by accident.

The humans are demons
They have been trapped in bodies for so long that they have essentially become animals. They slowly forget how to use their inherent magic; now they resort to summoning other demons to do it for them.
 * No John, YOU are the demons!
 * That header better not have been a set up for that joke.

The Demons are all a Hive Mind.
That's why they are in so much pain when they are ripped from their world. They're all part of a larger colony that mingles freely and chaotically.
 * Pretty much the exact canonical explanation.

The Demons are related to Hollows
They eat people's essences (souls), can devour each other to get stronger, and are veeery tough to kill without the right methods. Sound familiar? Some of their spells could even be related to Hollow abilities - Detonations = Cero, for example.

The "demons" are all a single Eldritch Abomination.
The consciousness within the Other Place was tapped into by humans completely by accident. The humans expected demons and monsters, with terrible forms and long names, to appear; the entity of the Other Place thus took forms and made up names. Bartimaeus, Farquarl, Queezle, and all other spirits are simply extensions of this entity, pseudopods of one big body.

Faquarl is an afrit
Well, how else could he defeat four freaking over powered djinns while in Hopkin's body and with a freaking knife in a Curb-Stomp Battle? It is alto stated that Bartimeus is a fourth-level djinn, and yet Faquarl completely outclasses him. It seems rather unlikely that just three "levels" make a difference so huge.
 * Well, Bartimaeus did once claim to have taken on six djinni at the same time. He was talking to Ptolemy at the time, too, which substantially raises the likelihood that he was actually telling the truth. A djinn's power level probably has a very great deal to do with the circumstances, precisely who's going against them, what condition they're in, how mad they are, etc. etc.