Castle in the Air is Diana Wynne Jones' sequel to her amazingly awesome novel Howl's Moving Castle. It was originally published in 1990 (four years after Howl's Moving Castle). At first glance, this novel doesn't sound like a sequel--it sounds more like a companion book at best--but I promise it does explain more about Howl and Sophie, just not right away and not, perhaps, in the most obvious way.
That said, this story is set in the Sultanates of Rashpuht a land far to the south of Ingary (where Howl and Sophie make their home). Instead of a land akin to King Arthur and Merlin, Rashpuht is much more likely to harbor Aladdin and other desert-dwellers. This change in setting, along with a new protagonist, make for the most dramatic differences between Castle in the Air and its predecessor.
Abdullah works as a carpet merchant in the city of Zanzib. Abdullah's stall may not be as prosperous as his father's first wife's relatives would like, but Abdullah can't stand most of them so he doesn't worry too much. What really bothers Abdullah is the fact that he's selling carpets at all. Abdullah is convinced there is more to life and spends a good deal of his time daydreaming about what his life could be like if, say, he were a prince who had escaped bandits and disguised himself as a carpet merchant before he found his true love.
All in all, the young man doesn't give his daydreams much thought until he is sold a mysterious carpet. With the carpet, Abdullah finds that all of his dreams seem to be coming true with alarming accuracy. Whisked to a magical garden, Abdullah meets and falls in love with the beautiful and intelligent Flower-in-the-Night only to have her abducted by an evil djinn. So begins Abdullah's adventure as he and his carpet set off to rescue his true love.
This being a novel by Diana Wynne Jones, the plot is filled with charming twists and enjoyable characters throughout. The other great thing about this novel is how much Jones fleshes out the world she introduced in Howl's Moving Castle. As the novel progresses, readers learn more about the relations between Ingary, Rashpuht, and Strangia (a land that becomes important later, trust me). At the same time, Jones also creates a completely new set of customs and even a new diction for her Rashpuhtian characters which gives the novel an impressive depth.
I don't know if this was the intended effect but, even though both novels are written in English, this change in diction also creates the effect that the characters here speak a different language and that, on some level, their customs would be very foreign to those found in Ingary. One of Jones' best inventions is that buyers and sellers in Zanzib always speak to each other "in the most formal and flowery way." This habit creates a lot of conversations that function on a variety of levels much in the same way body language can add to an exchange. For example:
"It is possible that my low and squalid establishment might provide that which you seek, O pearl of wanderers," he said, and cast his eye critically over the stranger's dirty desert robe, the corroded stud in the side of the man's nose, and his tattered headcloth as he said it. "It is worse than squalid, might seller of floor coverings," the stranger agreed.
Exchanges like this appear throughout the novel and make it really enjoyable to read. At the same time this type of double talk suggests that Abdullah is a shrewder narrator than Sophie might have been at the start of the novel. Abdullah doesn't always know exactly what's going on during the novel, but he always tries to make sure he comes out on top (or at least not on a forty foot pole).
On its own, Castle in the Air is a lot of fun as far as fantasies go. Read in combination with Howl's Moving Castle and House of Many Ways (Jones' latest novel featuring Howl and Sophie, released in 2008) this book is excellent.