Thank you to Purplepixie for sending me this wishlist book for my birthday.
Julia and her brother, Paul, are spending a day by the beach when they discover a midden heap. They climb to the top, where Julia discovers a rag doll in a ball of feathers hidden in a nest. It turns out to be a powerful and evil witch, and it possesses Julia and spirits her away through a pyramid of fire. Paul manages to follow them and he finds himself in a strange country desperately unprepared for the return of the Ragwitch. At first, he meets the May Dancers, who after questioning him, lead him to the edge of the forest and set him free. The people of the land aid him as he searches for a way to free his sister, action following at every turn, as both Paul and Julia battle the Ragwitch; Julia from within Her, and Paul from outside. Paul collects several mystical objects from the powerful Elementals, before meeting with the Patchwork King, who forges for him a needle spear in order to kill the Ragwitch. There are no strong fighters to help Paul, no saviors for him and he must find his own way. Because Paul is no hero, his war is one of bravery and brains, not brawn. Julia tells her own story from the mind of the Ragwitch. Although she is much more courageous than Paul, her war is one of the mind, resisting the power of the Ragwitch from within the witch's body

A brilliant read, typical sinister story from Garth Nix.
This is now going into the Paranormal VBB on Bookobsessed.