Disclaimer: If you want a real, (relatively) unbiased, critical review, wait until I'm not writing about my favourite series of all time.
The book which spawned my favourite film, Harry Potter and the Half Blood Prince is fantastically refreshing considering that it's the sixth book in a series. Despite the undeniably darker tone of the last three books, this one stands out to me as one of the more enjoyable, not just because it uses snarky, well placed humor, but it's also full of relationship development and sees a lot of ancillary characters come into their own. The overarching guidance of Dumbledore begins to strip away as we see Harry begin to realise his own destiny as the "Chosen One." Additionally, the plot line of the Half Blood Prince runs delightfully parallel to the Voldemort-is-rising arc, but gives the book that youthfulness associated with attending school and continuing educational development. By contrast, the events in the Deathly Hallows seem very much more mature, and I believe it is that linkage to school-days which Half-Blood successfully ties in with the wider plot lines.
Carrying on from the misgivings Dumbledore reveals in the end of the Order of the Phoenix, this year at Hogwarts sees a closer mentor relationship evolving between Harry and Dumbledore. It appears now that Harry is growing more emotionally mature and aware, and Dumbledore fosters this by letting him engage in the theories he holds about Voldemort and his past. Worlds apart from the relationship we saw in the first four books particularly (which was one of teacher/student and very much bent on protection and nurturing), we now see Dumbledore treating Harry as almost an equal, in the sense that even the conjecture that Dumbledore has regarding the Horcruxes is not withheld from Harry. We know, of course, that there is more to the story that Dumbledore doesn't let on in this book, but it still shows a marked development in their relationship, and really binds Harry as "Dumbledore's Man." Additionally, this engagement gives off the notion that Harry is succeeding Dumbledore as the vanquisher of Voldemort. Through Dumbledore's tutorship, Harry is being adorned with the weapons which will enable him to take on the Dark Lord after his departure from Hogwarts.
In terms of emotional development and maturity, we see two very stark contrasts appearing in this book. Harry, who spent most of the Order of the Phoenix struggling to close his mind, control his emotions and being tortured by Umbridge due to his frequent angry outbursts, turns a corner here. Harry is a great deal more mature here. He doesn't take Snape's bait when he is forced to do detention looking through his father's files or when he is late to the start of term feast, he doesn't lash out at his friends when he feels the slightest bit misunderstood, and he doesn't appear to be as concerned with what others think of him (i.e. taking Luna to Slughorn's Christmas Party). He is, in turn, given far more respect and spoken to more openly by his elders and teachers, and even manages to strike up a relationship with Ginny, miraculously without offending Ron (whereas he was virtually incapable of speaking coherently to a girl in book 5).
By contrast, we have Draco Malfoy, a character who has seemingly had a personality transplant from Order of the Phoenix. No longer the cocky, arrogant and proud figure he cuts in the first five books, Malfoy now suspiciously draws back into the shadows. He is given a task which he is incapable of successfully carrying out, and it takes its toll on his demeanor. Unpredictable, depressed and conspicuously absent from many of the day-to-day activities is a remarkable change for Draco, who usually appears as the relatively benign (although annoying) antagonist. Unlike Harry, who has gained the respect and trust of others, Draco is shown at the very first part of the this book is mollycoddled by his mother and Madame Malkin against his wishes, before being tailed by Snape, and then later taunted by the Death Eaters. His mission is clearly designed not to assist his development but to punish his father.Voldemort, a serial loner and uninterested in the development of others, shows Draco a very different leadership and mentor-ship than Dumbledore does to Harry and it makes for a fantastic contrast, building elements of all four characters.
In terms of character development and backstory, surely the most interesting would have to be that of Tom Riddle/Voldemort. The delving into various memories regarding the man who would become the Dark Lord is a fascinating venture, and absolutely crucial for peeling back the layers of fear surrounding him. The memories of Riddle as a child display his tendencies for being alone, his firm belief that magic could cure all (and conversely, that anyone who was prone to human weakness or mortality lacked magical power) and his overarching fear of death. He is shown to be cunning, uncannily intelligent and goal-driven, skills which proved to serve him well in his ultimate pursuit of power, and to master his own human fragility. And yet, we begin to understand what Dumbledore is on about when it comes to Voldemort's weaknesses - he underestimates others, particularly those whom he perceives as beneath his skill and intellect. He is incapable of opening himself up to others, meaning that he can never develop real relationships or true friends. He consistently fears his own weaknesses, that he is human, that his body can be destroyed, and that he could be rendered weak.
The power of words is also incredibly potent as a theme throughout this novel. The use of names - Tom, Voldemort, Riddle, He-Who-Must-Not-Be-Named and the Dark Lord all refer to the same individual, and yet it is indicative of power relationships between him and all others. Fearing normality and mortality, Tom Riddle rejects his name as one which symbolises not only his muggle father, but because it isn't "special" and that "there are a lot of Tom's." He clearly has a desire to be extraordinary, since being ordinary is something which ought to be feared.
Dumbledore, who doesn't fear Voldemort and in fact, even in death, is revered as being a greater wizard, calls Voldemort by Tom Riddle - indicative of the power relationship between them. On multiple occasions, he refuses to call Voldemort by anything else, and thus solidifies that power play with what Voldemort perceives as open disrespect. Harry, who doesn't fear Voldemort, but nonetheless lacks the power relationship that Dumbledore has over his foe, calls him by his fashioned name - Voldemort. His peers, who fear Voldemort and feel as if they have to (in fear) respect his power, refuse to say either his real name, or his fashioned name. Finally, those who revere and worship, call him the Dark Lord - a clever play on perhaps a quasi-religious zeal which his followers afford him.
Ginny enjoys a great deal of character development in Half Blood Prince also. Refusing to let her older brothers dictate her relationships with others, she emerges as yet another finely crafted female figure. Loyal and self-aware, Ginny seems far more mature than her age suggests, and perhaps this decisiveness is what Harry finds attractive in her. Nevertheless, I find it difficult to buy the Ginny-Harry relationship - predominately because prior to this novel there was really no romantic ties to Ginny at all, Harry thought nothing of her when he was chasing Cho Chang, and little attention was paid to her character aside from the role of damsel in distress in Chamber of Secrets. It sort of appears out of nowhere, the feelings which Harry suddenly suffers in relation to his new girlfriend - jealousy, anxiety and affection seem a little contrived and it's possibly the only thing I don't like in the whole series. Personally, I think Harry would be much better suited to another, or the relationship needed to built slowly, with far more interaction and interest. Put bluntly, Harry just didn't seem to care much for what Ginny did prior to this book, and now, it's suddenly all he can think about...when he's not thinking of Draco Malfoy that is.
"Harry, however, had never been less interested in Quidditch; he was rapidly becoming obsessed with Draco Malfoy."
"You're becoming a bit obsessed with Draco Malfoy, Harry."
- the two lines in Half Blood Prince which prove Drarry is real.
The end of this book of course sees the death of Dumbledore - an arguably extraordinarily pivotal character. It also sees Rowling depart from her plot device of using Dumbledore to explain the more intricate parts of the mysteries behind Voldemort's power and the relationship he has with Harry. It in some ways is handing the reigns over to the trio to now embark on that discovery alone. Without the moral guidance of Dumbledore to guide him, Harry now sets out on the path of hunting horcruxes a little blind. Whereas we always had a feeling that Dumbledore would be there to save Harry, it is now evident that Harry must take the responsibility for his own destiny and the power which he holds against Voldemort.
Dark, sad and deeply foreboding, the last pages of this book differ very much from the start of it. It indicates the reader stepping into the realm of the Deathly Hallows where much more is at stake, and the dangers are far higher.
Do I need to say it? Five stars.