SPOILER ALERT! DISCUSSION OF Harry Potter and the Half-Blood Prince! Well, they certainly do get darker. In this one, good old Hermione persists in her research, and Harry is absolutely-adolescent-certain of who the good and the bad are. Dumbledore dies (but what about that phoenix on his funeral pyre? And his knowledge of Horcruxes?), and Snapes is demonstrably a Death Eater (but remember—Dumbledore died believing in him!)
Harry's youthful determination to go for a soldier as it were is insightful-- after all, it is the human tradition for the young men, physically strong and most disposable, to go do battle for the Race. You can, if necessary, get a new generation with a few surviving men.
This book, though, I really felt Harry's destitution: he suffered through his childhood, a veritable Dickensian childhood. The loss of whoever loves him—so far Ron and Hermione have made it through alive-- but the rest are getting picked off. Was he, then, bred as a fighting-against-evil machine?
Rowling has done something interesting here. I keep thinking of my friend who is a senior at Columbia High School. I asked her if she likes the Harry Potter books, and she says she loves them-- except for the final book! This bodes ill for Harry's future.