I'm only to the part in the book where Harry finds out his potions book is 50 years old(page 250300 something), but I think I have figured who the Half Blood Prince is. I think that it is Voldemort. In CoS we learn Riddle(Voldemort) attended Hogwarts about 50 years ago, and in this one, we learn that he is half-blood. He was also a very bright student. Obviously the prince was. Anyway, this is who I think the Half Blood Prince is, so I guess this can only be confirmed by reading. Did/do any of you have/had theorys on who the HBP is?