I learned a new world today: penultimate. The second to last in a series. Good to know. Likely I'll forget it, but good to know for now.
Especially as Harry Potter and the Half-Blood Prince feels like a penultimate book. There are fewer flashbacks and recounting of past details, as if there is an assumption (a fair one) that few readers are likely to pick up this book without having some similarity with the previous story. And while there is a definite climax, it still feels like a stepping stone to the grand finale. I'm not complaining of this, just noting that the feel of this book is quite different.
Also differentiating the book are a few other style choices. In particular, Harry appears later in this book than in any of the previous books (he's not even in the first two chapters) and thankfully little time is spent on the Dursleys. And while Voldemort doesn't appear in the flesh himself, his presence is definitely felt and having him held back makes the reader both dread and look forward to the final showdown yet to come. Unlike the last book in which some readers complained about Harry's moodiness, he seems more focused and matured in this book.
My daughter took Dumbledore's death pretty hard, but her fury towards Snape gave her something else to focus on. She's too nice to admit it, but you can tell she wants blood. As for me, Harry Potter and the Half-Blood Prince is neither my favourite, nor least favourite in the series, but it felt necessary.