'In a brief statement on Friday night, Minister for Magic Cornelius Fudge confirmed that He Who Must Not Be Named has returned to this country and is once more active. "It is with great regret that I must confirm that the wizard styling himself Lord - well, you know who I mean - is alive and among us again," said Fudge.' These dramatic words appeared in the final pages of Harry Potter and the Order of the Phoenix. In the midst of this battle of good and evil, Harry Potter and the Half-Blood Prince takes up the story of Harry Potter's sixth year at Hogwarts School of Witchcraft and Wizardry, with Voldemort's power and followers increasing day by day ...
This is truly the point at which the story matures—in many senses. While the previous books retained their youthful innocence, this book solemnly lifts the Wizarding World to a whole new level. It is a world apart from the rather half-baked, borderline first books, which relied too much on clichees.
Book six is the maturity of the story: for the epic mystery that is finally lifted; for the gloomy prospects ahead. But also for Harry himself, as he finally gets drowned in teen drama. And lastly, it's the maturity of Rowling's writing. Her earlier books were nowhere near these later masterpieces. She has definitely learned a lot in the process of writing five books and here, in the sixth book, it finally shows.