Help, I can't remember this book, school-age, sci-fi?

"John Christopher is perhaps best known for his Tripods Trilogy - The White Mountains, The City of Gold and Lead, and The Pool of Fire, plus its prequel When The Tripods Came; and also his Sword of the Spirits Trilogy - The Prince is Waiting, Beyond the Burning Lands and The Sword of the Spirits. His most recent title was A Dusk of Demons."