The O2

Harry Potter's sixth year at Hogwarts turns out to be quite the exciting year. First off is the arrival of a new teacher at Hogwarts, Horace Slughorn, who is a bit more useful to Harry than he realises. Next, Harry obtains a Potions book which used to belong to the very mysterious Half-Blood Prince. Harry finds that the Half-Blood Prince's ancient scribbles are written along the margins of almost every page, giving Harry advice on how to improve on his Potions work, and also teaching him a few helpful (and dangerous) spells along the way.