Inspired by this question I was wondering why the Half Blood Prince left his book as a class copy. One would think that it was precious to him, since he invested so much in it. He made up potions and spells up by his own. I would expect him to keep it to himself. Both, because of the personal value of it, and to prevent others to (as happened at last) use his work (misguided and/or against him).
Is there any canon explanation why his book was left as class copy?