When I initially read Harry Potter and the Half Blood Prince, I remember disliking the story. After rereading it, I can say I've enjoyed it a lot more. This book definitely represents the point where the series becomes much darker.

In addition to its horrifyingly sad ending,

much of this story seeks to wipe out the magical excited feeling readers associate with Hogwarts by painting the world as a significantly darker place after the events of the previous two books. Additionally, Rowling dedicates a lot of time to developing romantic relationships between members of her cast in this book. Although I don't believe Rowling is the most skilled at these parts of her story (I found Ron's constant opinions about Ginny's love life to be very irritating), there are some really satisfying moments between characters here. The titular character is also incredibly well developed in this book and may be a highlight of the series for me.

While this book is really dark, it is worth noting that Voldemort plays an incredibly minor role in the story. It's telling that some of Rowling's best works are the ones where Voldemort is not involved.