The story
With the return of Voldemort now impossible to deny, the sixth book shifts from proving that the threat exists to understanding how it might be defeated. Dumbledore begins showing Harry a sequence of memories that reconstruct Tom Riddle's childhood, ambitions and transformation into Lord Voldemort.
At the same time, ordinary school life continues in a more fragile form. Harry finds an old Potions textbook covered in notes by a mysterious 'Half-Blood Prince', relationships change and Draco Malfoy is occupied with a mission that gradually becomes impossible to ignore.
The book provides the key conceptual bridge into the finale. The history of Horcruxes, Voldemort's choices and Dumbledore's strategy gives the final book its objective and turns earlier details into parts of a much larger pattern.