This is the second of the Septimus Heap books. Magyk was reviewed here. Spoilers for it ahead, though not, I hope, for Flyte.
It opens with a prologue depicting Septimus's oldest brother Simon making a Darke bargain out of resentment of Septimus's Apprenticeship. (This was brushed on at the end of Magyk.) And then it cuts forward a year. Septimus is enjoying his work, and staying inside on a lovely day because he had had quite enough outdoors while in the Young Army. But Marcia, a little bothered by a Darke Shadows that had attached itself to her, sends him off to see his mother and Jenna, since it's his and Jenna's birthday -- and Simon arrives, riding a dark horse, and carries off Jenna.
No one believes Septimus. They think it's just a prank. So Septimus, with some help from his friend Beetle, and his brother Nicko, sets out to find her. They find some help in the forest, and someone Septimus had never thought to see again. And the story winds out, about a house with many dolls, a circus, lots of unseasonable ice, hidden bones, a Flyte charm, the stone that Jenna had given Septimus in the last book, and the Dragon Boat.
As in Magyk, the story is resolved entirely within the book, though some things are picked up on, and will continue. It ends with a list not of what happened to various minor characters, but with how they had ended up there. Very wittily written.