Ptolemy's Gate (The Bartimaeus Trilogy, Book 3)

Three years have passed since the magician Nathaniel helped prevent a cataclysmic attack on London. Now an established member of the British Government, he faces unprecedented problems: foreign wars are going badly, Britain's enemies are mounting attacks close to London; and rebellion is fermenting among the commoners. Increasingly imperious and distracted, Nathaniel is treating Bartimaeus worse than ever. The long-suffering djinni is growing weak and vulnerable from too much time in this world, and his patience is nearing its end.