I also thought this book was much better than the last one. It seems that every author has a "fill" book that doesn't go smoothly from one point to another without excess padding, and IMO, OOTP was JKR's "fill" book.
I guessed early on who the Half-Blood Prince was, and am now more conflicted about Snape than ever. I thought the textbook was older than Harry's parents' generation, of which Snape was one. And someone who knew the spells and devised the charms that Snape did would have had much greater power to assist V than what Snape did. Think of how Snape threatened Quirrell in the first book, when Harry could have been done away with at no suspicion to himself. And I still think that D was/is smarter than to trust Snape on the basis of Harry's reasoning. . .
Just some random thoughts.
It apparently belonged to Snape's mother, who had been a student at Hogwarts, as well. Her last name was Prince.