"The two most abundant things on the planet are hydrogen and stupidity."
~Harlan Ellison

You may be onto something, AspenUSMC. I checked this out last night. The chapter art for the "Potions Master" chapter isn't identical to the book art for HBP, but they both do show books with something (steam? magic?) coming out of them!

A friend of mine finished HBP before I did, and he was having a hard time not spoiling me as I read. At one point, I actually did say something like, "good grief, what if it's Snape?" while guessing the identity of the Half-Blood Prince. I later eliminated him when I found out about the age of the book. Also, I believe Snape was mentioned in a group of wizards that were supposed to be pure-blooded. I guess that was the proverbial red herring.