Rowling has set Snape up to be the biggest, most misunderstood hero in the history of literature. I imagine a final scene in Book 7 where Snape is at last vindicated, when Harry realizes all he has done to protect him despite everyone’s efforts to bring him down.
Harry Potter and the Half-Blood Prince is a very good book, worth the wait, but a little lighter in detail than previous installments. I think my favorite books are still The Prisoner of Azkaban and The Goblet of Fire.
Rowling gave us a new clue we didn’t have before: Lilly Potter was excellent student of potions, whereas prior to the HBP we only knew her as an excellent charms student. The idea she would’ve been Snape’s frequent classmate opens up some intriguing possibilities.
The famously unreliable Trelawney has a couple of accurate readings (while conscious, too): once when she unknowingly stands close to Harry in the hallway, and the other when she predicts Dumbledore’s death in the tower.
Dumbledore told Draco that he could fake Draco’s death and hide him more completely than anyone would believe possible. Is Dumbledore faking his death to hide more completely? Is he going to pull a Gandalf?
Dumbledore appeared weaker and weaker throughout the book. He told Harry the horcrux that blackened his hand almost killed him and that it was Snape, not Madame Pomfrey, who saved him. What if the wound was fatal and Dumbledore went to Snape to prolong his life long enough to give Snape the ultimate proof of loyalty to Voldemort?
Snape didn’t touch a single professor after his betrayal of Dumbledore, despite many opportunities. His complete focus was getting the Death Eaters and Draco out of the castle as quickly as possible, and yet with his help and the element of surprise he could’ve decimated everyone.