The Journey by Kathryn Lasky
This is book #2 of the Guardian of Ga’Hoole series. In this book, Soren and his friends Gylfie, Twilight, and Digger band together to search for the legendary Great Ga’Hoole Tree. They have various adventures in their search and learn from a dying Barred Owl that there is something worse than the evil they have already encountered at St. Aggie’s. Unfortunately, the Barred Owl breathes his last before he can explain to them what that evil is. Eventually, the foursome find the Great Ga’Hoole Tree, receive training and are assigned to chaws according to their specific skills. Mrs. Plithiver comes along for the adventure and finds her gifting at the Great Ga’Hoole Tree as well. Soren gets assigned to the double chaw of weather interpretation and colliering under the instruction of Ezylryb, an old, strange and partly deformed Whiskered Screech Owl. At the end of the book, Soren’s younger sister Eglantine is rescued but she isn’t the same Eglantine that Soren remembers. Somethng sinister has happened to her and the other owlets that were found with her. The wise owls of the Great Ga’Hoole Tree have never seen anything like it and are very concerned that some new evil has sprung up for them to counter. At the very end, Soren’s mentor, Ezylryb, mysteriously fails to return from a mission. Soren feels in his gizzard that he must find Ezylryb and discover what has possessed his sister and the great number of other owlets found with her.