The Real 'Half-Blood Prince'
In the 6th book, Harry Potter and the Half Blood Prince, Harry finds a potions book in the back cabinet of his advanced potions class, with notes beyond the smarts of even Hermione Granger. These notes help Harry to strive in his potions class, becoming the favorite of new potions master, Professor Horace Slughorn. The book contained not just help on potions, but spells created by the previous owner. Not only that, they signed the inner cover, but as a code name: "Property of the Half Blood Prince."
