Harry Potter and the Half-Bloof Prince

Harry Potter accompanies Albus Dumbledore on a trip to persuade retired Hogwarts professor Horace Slughorn to return to teaching, before arriving at The Burrow.
At Hogwarts, Snape is named the new Defence Against the Dark Arts instructor, with Slughorn becoming the Potions teacher. Harry and Ron are able to enrol in his class, but Harry's borrowed book has notes and corrections that yield better results than the textbook's instructions, winning him a vial of Felix Felicis, a luck potion, on his first day in class. The book formerly belonged to "The Half-Blood Prince".
Harry becomes suspicious of his rival Draco Malfoy, after overhearing several of his conversations, and when Katie Bell is almost killed while attempting to deliver a cursed necklace under the influence of the Imperius Curse. Meanwhile, Ron is displeased with his younger sister Ginny dating anybody, forcing Harry to ignore his attraction. After an argument with Ginny in which Ron's dating inexperience is made clear, Ron takes up with Lavender Brown, creating a divide between him and Hermione. This rift is only mended when he accidentally consumes a love potion attracting him to Romilda Vane, and then poisoned mead which almost kills him.
Dumbledore privately tutors Harry, using his Pensieve to show Harry memories of Voldemort's past. Dumbledore asks Harry to retrieve a key memory from Slughorn regarding Slughorn's conversation with a sixteen-year-old Tom Riddle. Harry uses some of his Felix Felicis to acquire the memory.
The memories reveal that Voldemort splintered his soul into six fragments attached to objects called Horcruxes, while leaving a seventh piece in his body, in order to achieve immortality. As long as any of the Horcruxes exists Voldemort cannot be killed. Two Horcruxes have been destroyed - Tom Riddle's diary by Harry in the Chamber of Secrets, and Marvolo Gaunt's ring by Dumbledore. Three of the remaining Horcruxes are a locket formerly owned by Salazar Slytherin, a cup formerly owned by Helga Hufflepuff, and the snake Nagini. Dumbledore speculates that the sixth unidentified Horcrux may be something associated with Gryffindor or Ravenclaw, as Riddle would have wanted something from each of the houses of Hogwarts, which he regarded as his home.
After seeing Harry cast an unusual curse, "Sectumsempra" (a dark curse which leaves deep gashes) against Draco Malfoy, Snape attempts to confiscate the Half-Blood Prince's Potions book, but Harry gives him Ron's copy instead and hides his copy in the Room of Requirement. Harry gets detention from Snape and misses the Quidditch finals, but Gryffindor wins the Cup. During the victory celebration Harry spontaneously kisses Ginny, and with Ron's grudging approval they begin dating.
Dumbledore locates another Horcrux and asks Harry to accompany him to destroy it. They travel to a cave and retrieve a locket, but Dumbledore is weakened after drinking a potion to obtain the Horcrux. After they return to Hogsmeade, they see the Dark Mark over the Astronomy Tower at Hogwarts and hurry back to the school. Dumbledore and an invisible Harry are confronted atop the tower by Draco Malfoy. Draco admits that he was behind two attacks on Hogwarts students, with both objects intended for Dumbledore, whom Voldemort had ordered Draco to kill. Dumbledore offers Draco protection from the Death Eaters, and Draco cannot bring himself to kill Dumbledore, even with the urging of fellow Death Eaters once they arrive. Instead, fulfilling his Unbreakable Vow, Snape kills Dumbledore. Harry, enraged and freed from immobility by Dumbledore's death, pursues Snape, who reveals that he is the Half-Blood Prince and fends off Harry's attacks (and Buckbeak who joins in to help Harry) until he gets outside Hogwarts grounds and Disapparates.
Harry recovers the locket from Dumbledore's body, only to discover it is a fake left by someone with the initials R. A. B., who stole the real Horcrux and left a note about his opposition to Voldemort. McGonagall tries to question Harry about what he was doing with Dumbledore and he refuses to say. McGonagall calls a meeting of the Heads of House (with Slughorn taking Snape's spot) to decide what to do. They decide to close the school, but at Harry's suggestion, agree to wait until Dumbledore's funeral so the students can say goodbye. The Minister arrives and McGonagall lets Harry leave so he can avoid the Minister. The school year ends with Dumbledore's funeral. He and his wand are buried in a tomb beside the lake on Hogwarts's grounds. Harry vows not to return to school but to hunt for the remaining Horcruxes instead. Harry breaks up with Ginny to keep her safe from danger during his quest, and Ron and Hermione pledge to accompany him.