Ranger's Apprentice #6 The Siege of Macindaw by John Flanagan

My eleven year old read this book to himself.
Sir Keren has overtaken Castle Macindaw.  It is up to Ranger Will and his friend Horace to release Araluen from Sir Keren's grip.  Held captive in the Castle Macindaw is Alyss, the one Will loves.
*Highly recommended by my 11 year old.
Warning: swearing
The library has this series labeled "young adult".