Harry Potter and the Half-Blood Prince
I read this again because i had forgotten some of the details, and that was annoying me as i thought about the book in the wake of the release of the fourth film. So, having reread it, i have a better grip on the details of the plot and some characters. The process has reinforced my belief that Snape cannot actually be still in the Dark Lord's camp; he is really too close to being a hero to be a villain. And Harry himself is becoming a less attractive character, or that is being revealed to me more than i understood in the previous books. Snape calls him a liar and a cheat, both of which are absolutely true accusations, and make Harry's actions questionable; does the end result justify unethical means? An old question which has been given different answers by different people; Harry would obviously answer that it does: His actions have proven many times that he has that Machiavellian slippery grasp of ethical logic; i wonder how Rowling would answer.
This was, obviously, some time ago, well before the final book in the series was published. Interesting to reread this now, knowing what we do about Snape and others.