I am now reading the sixth Ranger's Apprentice book in the series. It is called The Seige of Macindaw and it is by John Flanagan. It is about a young ranger who needs to stop a renegade knight'splans. The knight has already taken over the border castle and has been conspiring with the enemy. The ranger's girlfriend is held hostage by the knight, in the castle. I really like this book because there is a lot of strategy and action involved.