Ptolemy's Gate (Book 3 of the Bartimaeus Trilogy) by Jonathan Stroud, 2006

John Mandrake -- or should we call him Nathaniel? -- has grown in the ranks of power in magician-run London, and in popularity with the restless commoners, but he's more dependent than ever on Bartimaeus for demonic (and emotional) assistance. But a failed attempt to discover the identity of the magician fomenting rebellion leaves Mandrake and Bartimaeus vulnerable. Meanwhile, Kitty's been doing a little reading, and she and Bartimaeus have a few encounters of their own. Suspenseful, effecting a thorough transformation, with a staggering conclusion. Most strongly recommended, but read the entire series in order.