The Half-Blood Prince arms Harry with powerful tips, tools, and ideas that allow Harry to both coast in Potions class and to learn powerful magic. The irony here is that the Half-Blood Prince, we come to find at the end of the story, is most likely Professor Snape himself. The very magic that Harry soaks up and appreciates so dearly comes from the man who kills Dumbledore. Is the textbook planted in Slughorn's classroom? If not, why didn't Snape hold onto it? What exactly does the book teach Harry to do?