Harry Potter and the Half-Blood Prince is a character study; a respite from the almost non-stop action of the last five books and the action that must follow in the next one. Therefore, barely anything happens in this book. It even has the delightful audacity to begin with a very British comedy scene involving the [metaphorical] heartburn suffered by the Prime Minister of England when visited by the Minister of Magic.
Mostly we learn the semi-tragic history of Voldemort aka Tom Riddle. While there was certainly tragedy to his story, there does not seem to be an air of inevitability to his turning to evil.
Because not much actually occurred, there’s not much else to say without spoiling the plot. The only things I’ll add are that this book answered my question of why pensieves and verisatem potions are not the perfect solution to trial testimony in the Potterverse. Also, I had gone until last year without being spoiled on Harry Potter until someone was listing old spoilers like Citizen Kane, Planet of the Apes, The Sixth Sense, etc and someone thought it appropriate to lump Harry Potter into that list of mostly decades old spoilers. As I’ve matured as a consumer of stories in the past decade in which I’ve studied tropes and gained a better understanding of storytelling, I’ve come to the conclusion that with the exception of a very small subset of stories (like Whodunnits), a good story should not be able to be ruined by spoilers. The Harry Potter series is no exception (as it is a well-written series). Although I went into this first read-through (other than the first book and a half which I’d read a decade prior) knowing that a certain someone would kill a certain someone else (and even recently via a comedian learned it would happen in book 6), it did not diminish my enjoyment of the series or this book. Rather, it made me wonder about the circumstances under which it would happen. (view spoiler)[I must say that all my theories about Dumbledore (which I’ve mentioned in one or two prior reviews) were wrong. I’d assumed he’d turn out to be an even darker wizard than Voldemort and that’s the reason Voldemort was scared of him (hide spoiler)].
So a good book, but not my favorite in the series. It could easily have just been compressed a bit and made to be the first half of what is currently book 7 (which I’m about to start), but I think it works as a nice breather after so much action and, as I said before, the fact there must be much action coming ahead.