Flight of the Genie, Book 21 by Tony Abbott

The city of Ut is a city of secrets. Mysteriously, it only appears once every one hundred years. And to enter, you need the key, which is kept in the magical bottle of Ut. Eric, Keeah, and the others need to get inside the city to help a friend. The good genie, Hoja, is being held prisoner there, and he needs rescuing! They have discovered the secret of the magic bottle, and so they know they can get inside the city walls. But what they don't know is that Ut holds other secrets. Dark, dangerous secrets. Secrets that involve a certain sorcerer named Sparr...