This is the book that marks the beginning of the end. The pieces are slowly coming together, Dumbledore now has Potter by his side working to help learn the secrets to Voldemort.
The relationship between the three friends are fragile as ever as love begins to play a role in their actions.
The Half-Blood Prince may be the one book with the greatest of character build AND plot evenly balanced out. Mostly because this is the second to last book, all the pieces that Rowling has set up, now need to be put in the right places in order to advance us to the finale. It partly works. Everything is balanced, the twists are rightly timed, the turns are never forced. However, as far as relationships go, some do feel forced.
Before anyone complains, I don't hate that Harry is paired with Ginny or anything along those lines, for anyone. I just wished it had been built a bit clearer.
Unless I missed a piece early on, there's a moment where Harry suddenly begins to think about Ginny and it feels completely out of place. as though she had the thought from the beginning they should end together but forgot to really add that throughout.
It's small bits like that, that caused me to take off a star. The plot is solid, the characters are at their peak, but the relationships seem to be forced or just unnatural at times.