Guardians of Ga'Hoole #5: The Shattering
In the fifth book in this series, the war between an evil group led by Soren's brother, Kludd, and the owls of the Great Ga'Hoole Tree rages on. In the midst of war, Eglantine unwittingly becomes a spy for Kludd, leader of the Pure Ones (a group of evil owls). She is brainwashed by an owl sent by the Pure Ones to infiltrate the Great Ga'Hoole Tree. Her odd behavior eventually attracts attention, and Soren and his friends vow to find out what's wrong with Eglantine. They ultimately learn what happened and help her reverse the effects of the brainwashing.