What is the theme in Harry Potter and the Half-Blood Prince by J. K. Rowling?

Sense of identity is a big theme in this story. Harry and his friends are coming of age. Adolescence is difficult at the best of times let alone growing up in a boarding school for magic. As their time at Hogwarts comes to an end there is war inside and outside the castle walls. Harry and his friends must search for their own individual identities and purpose in life which includes life outside of Hogwarts domain.