The Dagger in the Desk — Jonathan Stroud

A thrilling new case for London's most talented psychic detection agency. In London, a mysterious and potentially deadly ghost is stalking the halls of St. Simeon's Academy for Talented Youngsters. It lurks in the shadows, spreading fear and icy cold -- and it carries a sharp and very solid dagger…The headmaster wastes no time in enlisting the help of ghost-hunters Anthony Lockwood, Lucy Carlyle, and George Cubbins. Can Lockwood & Co. survive the night and save the day? Don't miss this exciting Lockwood & Co. adventure, which includes a bonus Ghost Guide and a preview of The Hollow Boy, the next book in the series!