Speaking of magic spells, make that curses, Katie Bell is thankfully recovering from whatever curse was on that necklace. In time (not to sound like Dumbledore, but I do), I suppose we will find out who was the intended target of the curse. And maybe who sent it.
Snape has Katie's recovery under control we're told. As a double-agent truly working for Dumbledore (that's my current opinion; see my post on Chapter 2 of this book for why), he might be able to conjure a way to figure out who it was meant for and who sent it.
Harry's eagerness for his face-to-face "lessons" (no distance learning required at Hogwarts) will be interesting to follow. There is a heavy nature to these encounters with memories swirling about in the Pensieve that could be mentally taxing for Harry. But his curiosity always pushes him forward even if it sometimes betrays him.
Harry is convinced that Draco Malfoy is a death-eater-in-training. He might be, but it seems that Dumbledore would be attuned to the future of Draco based on his experiences with a young Tom Riddle.
I would be judgmental to accuse Dumbledore of having been naive with the fledgling and orphaned (at least by his mother) Voldemort. No doubt he believes he should have seen darkness coming in young Tom Riddle. I'm sure he's had his wand up ever since those days when a student like Draco shows up. He knows, I assume, what to watch for.
What are we to watch for? Who is the Half-Blood Prince? What more will the Pensieve reveal? Who is Snape's true master? As always, there are more questions than answers.
And I'll keep trying to answer them.