Harry Potter and the Half-Blood Prince is—by far—the standout entry in J.K. Rowling’s series of novels about the wizarding world of Hogwarts. With this novel, Rowling takes her saga from the sphere of entertainment for kids into the realm of serious literature. The story began turning darker with Goblet of Fire, but darkness is not the same as seriousness. Order of the Phoenix bridges the tonal transition from fairy tale darkness into adult literary serious, but Half-Blood Prince reads more like Steinbeck than like the Brothers Grimm.
The quality on hand here is one of purpose. The purpose is to lend Voldemort a humanity and sociological motivation that takes not quite fully out of the land of giants and elves, but manages to place him squarely within the more accessible world of class division and psychological compulsion. Voldemort is transformed from an abstract evil which can be given power merely at the mention of his name into a lonely orphan whose self-hatred and revulsion toward society is mirrored almost daily in the stories of real life villains with who readers can identify more directly.
The very title Half-Blood Prince indicates the overarching thematic concern of the novel and it is perhaps noteworthy that this becomes the first Harry Potter book to project its title so steadfastly into the mainstream world of real life cause and effect. The details about the past of Tom Riddle, Jr. and what made the angry young orphan into a wizard of devastating power rips the story of Harry Potter out of the magical realm where things happen that would defy all rational explanation and into the world of rejection and repression, class warfare and oppression and, especially, the way that the choices made by others in the past inexorably lead to decisions made in the present which impact people well beyond the scope of expectations.
Harry Potter and the Half-Blood Prince is also about making a complete whole out of incomplete fractions. The key to defeating Voldemort is divulged to lie in the various parts of his soul which Voldemort split apart. This splitting of Voldemort’s very essence is mirrored in the union of Voldemort and Snape as half-blood princes and in the union of Tom and Harry as orphans who choose different paths in response to parental influences and the lack therefore and in the union of Harry and Draco as boys flung against their will into the center of a battle between good and evil who both ultimately come face to face with the essence of their being that reveals their humanity. Also at play in the novels thematic richness as it relates to the titular concepts of halving is the significance that Rowling places upon how exploitation of one class by another impacts not just society, but the individual.
The richness here rises almost impossibly above how similarly important themes are treated in previous novels. Class division is not merely a plot mechanism that makes for a convenient backdrop. Rowling does not make it easy on herself or her readers by fueling Voldemort’s evil as a natural reaction to a pureblood member of the upper class looking with smug superiority upon the inferior classes below. Voldemort is driven by a compulsive mania fueled by hatred of the impurity inferior classes that arises out of a hatred for being a member of that very same inferior class. The irony of Voldemort being exactly that thing which he uses to work up his followers into a frenzied, uncritical support is devastating not just because it is unexpected, but because it resonates with reality. Readers whose eyes have grown tired after reading one-hundred pages can turn on the TV or log onto the internet and see this exact same thing play out in the real world. The palpable reality of people like Tom Riddle, Jr. rising to power by gaining the support of followers through the act of projecting their own flaws onto opponents lends Tom Riddle’s transformation into Voldemort all the more believability necessary to allow him to step fully out of the land of fairy tale evil and into the real world of the Muggles learning that Voldemorts are not manifestations of magic, but symptomatic of society's failure.