Or: six down, one to go.
Harry returns to Hogwarts, finding the darkness/whimsy ratio has gone up again for the fifth straight year. And he's busy. In addition to normal coursework, Harry's captaining his quidditch team. Headmaster Dumbledore asks Harry to meet with him for tutoring, which primarily consists of trips down the memory hole, researching how Dark Lord Voldemort came to be, how he survived his first encounter with Harry, and how he could possibly be defeated for good. In the meantime, Voldemort and his minions are engaged in a foul plan to strike within Hogwarts itself; Harry's nemesis, Draco Malfoy, is clearly up to no good; the seemingly slimy Severus Snape is apparently pledged to help him out.
Fortunately, Harry is helped out in his weakest subject, Potions, by finding a textbook heavily annotated by someone billing himself as the "Half-Blood Prince". Hermione sees danger in this, but Harry heedlessly takes the free advice the Prince offers.
Oh, yeah: somebody big dies at the end. (No spoilers here, but how can you not know?) Ms. Rowling is clearly setting things up for the final book.