I finished Harry Potter and the Half-Blood Prince yesterday afternoon, just as I thought I would, and I have a couple of spoilerish thoughts. So here's some empty space to protect you who would remain un-spoil'd:
(remember: spoilers below!)
OK, that should do it. From this point on, I make no attempt to conceal spoilers. I shall not be cagey in any way. I'm not going to refer to "the character who stunningly bumps off a beloved old favorite", I'm going to refer to them by name. Got it? Good. That was your final warning.
So Snape killed Dumbledore. Holy Shit, who saw that coming?
Well...I did, kind of. I don't think there was ever any surprise that Dumbledore died; I figured if he didn't die in the last book, he had to die in this one. The powerful mentor always has to be shown the door. Obi Wan Kenobi and Yoda. Quint in Jaws. The old bartender in Cocktail. Gandalf (in a way). Uncle Ben in Spiderman. (More on Spiderman in a moment, by the way.) I never once doubted that Dumbledore would die, and not just because J.K. Rowling knows her Campbellian story structure, which the Harry Potter series roughly follows, but because she knows another principle of storytelling:
Create likable characters, and then put them through pure and unadulterated hell, for the greatest heroes are those who stand up and do the right thing even after their world has been irrevocably destroyed.
That's what Rowling has done here: she's given Harry his baptism by fire, and almost pushed him down as low as he can go. (But note that he can still go down even further: Ron and Hermione are still alive.)
So no, I didn't find Dumbledore's death surprising at all. It would have been more surprising if he'd lived, frankly. And it wasn't really that surprising that Snape killed him, given Snape's Unbreakable Vow.
Here, then, is where I start theorizing. The question is, then, has Dumbledore's trust in Snape really been that misplaced all along, which Rowling seems to want us to believe? Or is there something even deeper in play here? I strongly suspect the latter. The book opens with Snape swearing to protect Draco Malfoy and submitting to an Unbreakable Vow to do so. The implication is that, by refusing to help Dumbledore, Snape is fulfilling that Unbreakable Vow.
But since Draco Malfoy isn't really in need of protection at that exact moment, it would seem to me that Snape's departure with Malfoy is the fulfillment of his Unbreakable Vow -- which would make his killing of Dumbledore something different. Perhaps the fulfilling of another Unbreakable Vow? One he has already sworn to Dumbledore? That would explain Dumbledore's unshakable trust in Snape: it would have nothing to do with Snape's motivations, or whether Dumbledore has judged Snape's character or anything of the sort.
So I suspect that when Harry was a baby, before Dumbledore placed him with the Dursleys, Dumbledore swore Severus Snape to an Unbreakable Vow in lieu of killing the former servant of Voldemort, which Snape somehow fulfilled by killing Dumbledore.
Of course, I could be entirely wrong; this is a half-baked theory, of course, and I'm not well-versed enough on the arcana of the Potterverse to flesh this out any more than I already have. We'll learn soon enough, anyway. (Or not soon enough, since I want Book Seven now.)
A few other thoughts:
1. They can't figure out who R.A.B. is? Come on!
2. I actually enjoyed all of the teen-romance stuff. Maybe it's because I'm a hopeless sap, but I found a lot of that stuff funny and true to the way I remember those stupid youth romances going. But that leads me to....
3. So, I wonder if J.K. Rowling's list of favorite movies includes Spiderman, because the last scene in this book nearly perfectly echoes the last scene of Spiderman. It takes place at a funeral, and after the service, Harry tells Ginny that he can't be with her because it will be too dangerous for her to be Harry Potter's girlfriend -- just like the way Peter Parker tells Mary Jane Watson that he can't be with her (although, if memory serves, he doesn't spell this out exactly).
4. Thinking back on Voldemort's/Tom Riddle's youth, I wonder if maybe Dumbledore isn't quite as wise as he seems after all: "Hi Tom. You certainly seem to be a sadistic creep and a criminal in training, but come with me and we'll teach you how to be a Wizard!" It kind of reminds me of another time a young kid with a trend to fear and anger was taught to use mystical powers, with this other kid also growing up to change his name to something more nasty sounding...it even started with a 'V'....
5. Where are the remaining Horcruxes? I really don't know, and I'm not confident enough to hazard any guesses. Rowling has, in the past, hidden big things in plain sight (Moaning Myrtle's identity, Scabbers being Peter Pettigrew); and at other times, she's done new things entirely (hinting strongly at a death in Goblet of Fire, only to have it be Cedric Diggory, the Potterverse equivalent of Star Trek's red-shirted security guys). I've read a few discussion threads with ideas aplenty, and each idea I see has me saying, "Hey yeah! That would be a great hiding place for a Horcrux!" But OK, I'll hazard one guess: I wouldn't be surprised if one Horcrux is sitting in the Dursley's living room.
Here's a really good spoiler-filled post on the book, and here's an equally-good discussion thread. Barring an announcement from Ms. Rowling along the lines of "I'm two-thirds done with Book Seven, and it will be out for Christmas!", I hope to re-read the entire series before Book Seven comes out. And I also intend to be as spoiler-free as I can possibly be for it, which will be quite a change for me after my "Give me the spoilers NOW, dammit!" approach to Star Wars for the last six years.
At the very least, she'd better not take as long to come up with Book Seven as George R. R. Martin has to come up with A Feast for Crows!