I spoke with one of my rabbe'im who is a recognized posek. He does not want to be associated with Harry Potter so I agreed not to publicize his name. He thinks that from the standpoint of the laws of Shabbos, it is permissible to cut the tape on the box, remove the book and read it. However, he does not necessarily recommend reading Harry Potter on Shabbos or during the week, and he points out that there are posekim who would rule strictly. But as he said, if this were a box of napkins that came in the mail, he'd allow it to be opened.