Harry Potter and the Half-Blood Prince is an excellent story and does quite a lot to offset how much I didn’t care for Order of the Phoenix.
First, Harry is not nearly as annoying as he is in Order of the Phoenix. Perhaps he’s growing up, perhaps Rowling realized that she’d perhaps overdone it and turned it down again. There are a few iffy moments around relationships this time around, but they’re much easier to read and feel much more natural, at least to me.
Second, things happen. Where Order of the Phoenix felt like it was spinning its wheels, Half-Blood Prince goes full tilt towards the end. After reading it, we know far more about Voldemort than we ever did before, including how he survived and what it will take to kill him. True allegiances are revealed and people die. So very much happens; it’s even more impressive that this book is actually decently shorter than the previous two.
And oh man the twists. It’s fairly well known now, but nevertheless, it’s fairly impressive how powerful the last few chapters of this book are. It’s something that likely had to happen for the sake of the story, but that doesn’t make it any less.
It’s not quite my favorite book of the series, but very close. Well worth the read.