I began to suspect that Snape was the HBP early on, since he was so adept at potions. When Harry realized that his book was fifty years old, I was even more convinced of Snape, rather than put off. It was a second hand book. For some reason, that memory of Snape's from his childhood, when he was watching his parents argue, made me think that Snape had been brought up in poor, squalid conditions. A hand-me-down textbook fit him.
OY! Only time will tell, and we've got some waiting to do!!!
Man, are we on the same track!
Let's ge to the 7th book already so we can see how it plays out!
Dumbledore wanted Snape on the astronomy tower for a reason that had nothing to do with his being ill (as I was thinking at the time).