"When Tobias and his friends were given the power to morph, they were also given an important warning: Never stay in a morph for more than two hours. But Tobias broke the time limit, and now he’s trapped in the body of a hawk — forever. When he discovers an important Yeerk secret, Tobias knows he has to do everything in his power to destroy it. But to do so, he’ll have to contend with a part of himself that’s wrestling for control. A part that isn’t human."
[This is the spoiler free discussion post for this book! If you want to see the spoilery version, head over to here for the one at animorphs. But that link WILL have spoilers for the whole series, so click at your own risk!]