On a unrelated note, a 'castle in the air' is a common idiom for an impossible or fantastic dream.
A children's fantasy novel by Diana Wynne Jones. This is the sequel, of sorts, to Howl's Moving Castle.
Later in the book you will find many of the characters that first appeared in Howl's Moving Castle, but in the beginning, this is just a story about Abdullah, an up-and-coming young carpet merchant living in the bazaar of Zanzib (somewhere in the Middle East). He is a good trader and his business isn't doing too badly, but he does tend to spend an inordinate amount of his time daydreaming. The story begins when a suspicious-looking stranger appears, trying to sell a dingy and threadbare flying carpet. Abdullah buys it, only to find that it will not fly once the stranger has left. Except that it does fly, once Abdullah falls asleep, and it carries him to a beautiful garden with a likewise beautiful princess (unfortunately named Flower-in-the-Night) wandering amongst the fountains.
The next day, Abdullah wakes to find that the carpet has returned him to his stall. Worse, his repulsive relatives drop by, announcing that they are ready to reveal the (supposedly long lost) prophecy that was made at his birth. Against his better judgement, Abdullah goes to hear the prophesy: "This son of yours will not follow in your trade. Two years after your death, while he is still a very young man, he will be raised above all others in the land." He also finds that his family has arranged for him to marry a couple of his step-mother's niece's nieces, to keep any wealth and power that might result from this prophesy in the family.
Fortunately, Abdullah bluffs his way out of this, in hopes of marrying his princess... only to have her snatched away by an evil djinn. As if this isn't bad enough, he suddenly finds that all of his daydreams are coming true, in disturbing (and suspiciously accurate) detail. He had imagined himself in the role of a long-suffering, perhaps even tragic, hero. Needless to say, it was more entertaining in the abstract. He stumbles miserably from one misadventure to another, hoping to somehow find Flower-in-the-Night again.
And here I draw the veil, in the theory that spoilers are called spoilers because they ruin stories. The Howl's Moving Castle cast doesn't come in until well after things are really underway, but they do play major roles towards the end.
This has been one of my favorite books for as long as I can remember, but upon rereading it for the umpteenth time, I am struck by how little this book is like most of the books I liked when I was a kid. There is too much background, to much description, and it's a little bit too sappy at parts. The story line also seems a bit jumpy; first it's about flying carpets and djinns, then it's about traditional western wizards and witches, then it's everything mixed together. I wouldn't recommend this book to most kids -- I would recommend Howl's Moving Castle, or Jones' Chrestomanci books, and I would assume that they would love them (who wouldn't?), and would thereafter read Castle in the Air, and probably love it too. But it is a little bit off.
As with most of Diana Wynne Jones' books, this could be considered comic fantasy, but not excessively so. It is enjoyable and occasionally funny, on about the same level as the Harry Potter books. It's fun! It's not silly, or full of jokes.
Originally published by Methuen Children's Books (UK, 1990), and Greenwillow Books (USA, 1991).