There is a door at the end of a silent corridor. And it's haunting Harry Potter's dreams. Why else would he be waking in the middle of the night, screaming in terror? .Here are just a few things on Harry's mind: .- A Defense Against the Dark Arts teacher with a personality like poisoned honey .- A venomous, disgruntled house-elf. .- Ron as keeper of the Gryffindor Quidditch team. .- The looming terror of the end-of-term Ordinary Wizarding Level exams. .…and of course, the growing threat of He-Who-Must-Not-Be-Named. In the richest installment yet of J. K. Rowling's seven-part story, Harry Potter is faced with the unreliability of the very government of the magical world and the impotence of