HP and the Half-Blood Prince - second thoughts

[Katrina]

So I just finished today reading through HBP for only the second time since it came out last summer. I loved it almost more than I did the first time. I came very near to crying at a few points.

One thing I particularly enjoyed was the name of the chapter after Dumbledore dies: "The Phoenix Lament." I hadn't thought about it before, but this is a wonderful ambiguity -- not only does Fawkes the phoenix lament in this chapter, but we also see members of the Order of the Phoenix lamenting.

Favorite moments in the book:

"But he understood at last what Dumbledore had been trying to tell him. It was, he thought, the difference between being dragged into the arena to face a battle to the death and walking into the arena with your head held high. Some people, perhaps, would say that there was little to choose between the two ways, but Dumbledore knew -- and so do I, thought Harry, and so did my parents -- that there was all the difference in the world." (Chapter 23, p. 512)

"'What do I care how he looks? I am good-looking enough for both of us, I theenk!'" (Chapter 29, p. 623)

"'This is ... not the moment to discuss it,' said Lupin, avoiding everybody's eyes as he looked around distractedly. 'Dumbledore is dead....'
'Dumbledore would have been happier than anybody to think that there was a little more love in the world,' said Professor McGonagall curtly[...]." (Chapter 29, p. 624)

And lastly, a few unanswered questions I have:
1. What was the spell that Snape used against Harry right before leaving? Harry has tried using Sectumsempra against Snape, who then reveals that he's the HBP; then Harry calls Snape a coward, Snape yells, and then he uses some spell against him, which causes Harry to get white spots in his vision. I kind of wonder if this is another of Snape's own home-made spells that no one else would know, and if it will be at all siginifcant in the last book.
2. What on earth was Snape's old Potions book doing in the cupboard, anyway? I can't help but think it was there for some purposeful reason. Why would he have taken all the time to write out important spells and potions information and then just leave it around someplace. Wouldn't he want to keep careful track of all that work?
3. Is there any deeper significance to Eileen Prince?

And, one final thought: I am still really angry that Harry would break up with Ginny so that Voldemort wouldn't use her to get to him, and then cave in when Ron and Hermione want to come along.