With Harry's occlumency lessons with Professor Snape a resounding failure, he begins lessons with Dumbledore instead. Together, they explore memories collected by Dumbledore in an attempt to better understand Lord Voldemort. 
 Meanwhile, Harry has found an old textbook filled with the notes of the previous owner - known only as the Half-Blood Prince. The notes help Harry excel in his Potions class, but after the encounter with the evil book in The Chamber of Secrets, Hermione has some reservations about using such a powerful book. 
 There was some interesting play with morality in this volume. Not to give too much away for the three people left on earth who haven't read the series yet, but Harry has to cause harm to someone he loves for the greater good. It's an extremely powerful scene that marks the tone for the rest of the series. 
 Seriously, I was bawling.