This is marketed as 'a sequel to 'Howl's Moving Castle', but the familiar characters from that book don't make an appearance (well, that we know of) till the very end of the book - so readers who are not familiar with that book will equally enjoy this. It's just sort of that for fans of 'Howl's Moving Castle', there's a bonus at the end!
It's an Arabian-Nights-flavored story of a young Middle Eastern carpet dealer who, against his better judgement, is convinced to buy an allegedly magic carpet. But when he falls asleep on the rug, he wakes to find himself transported to a garden inhabited by a beautiful young woman. At first, he believes it's only a dream - but the carpet's powers of flight are real, and soon he finds himself with an enraged Sultan to deal with, not to mention assorted bandits, a temperamental genie, and more... Familiar elements, but Wynne Jones' wit and style make the story fresh and entertaining.
This is my least favorite of the HMC books. It's basically an Aladinn re-telling with Howl and Calcifer jammed into the corners. It's nicely told, and to my surprise I re-read it all the way through, but I don't think it's fair to bill it as a "Sequel" to Howl's Moving Castle. It's a companion book at best, and it's really not about Howl and his gang. Nor does it have the wonderful all-original feel of the first novel, for the reason mentioned above.
That's not to say it's a bad book. The author simply did a good job of getting me to like the original bunch and I wanted to see more of them, not Flower-in-the-Night.