Bartimaeus Trilogy is epic.
It takes place three years after the events of The Golem's Eye - Nathaniel is the seventeen-year-old Minister of Information (or some such thing) now, and in the past years he's grown even colder and imperious and paranoid. In other words, he's become a typical (if brilliant and talented) magician. His career is precariously perched on the barely not-collapsing shoulders of the British Empire, and for the first part of the novel he thinks of himself as John Mandrake. You'll see what I mean when you read the book.
Our other human protagonist, Kitty Jones, is living the underground and illegal life; she had been officially declared dead at the end of The Golem's Eye, and she is now living two lives under false identities; by night she works in a bar where commoners often gather to talk about what they can do to change their situation and overthrow the government, and by day she's a magician's assistant (to an old magician who doesn't see eye-to-eye with the government), with the hopes of summoning and asking the assistance of one particular djinni.
Yes, she wants to summon Bartimaeus. They had an interesting conversation three years prior, but little does Kitty know that Bartimaeus isn't in good shape. At all. Everyone's favorite djinni and first-person character had been continuously summoned to Earth for about two years, and as a result, he's crankier and weaker and cheekier than ever. In this book, though, we finally start to learn about his past with Ptolemy (yes, the same one whose Gate is in the title), so that's fun.
Anyways, this is a very very good end to the trilogy; I swear, half the book is about the epic! showdown! at the end (also known as the "climax" of the trilogy). It's fun!
Oh, and I took the cover art from this blogpost. Yes, this ten-year-old can write reviews of comparable caliber to mine. (I kid! But it's cute.)