In this book, Harry and his friends are almost grown-up while their fight against Voldermort intensifies.
After receiving their results on the examinations following their 6th year at Hogwarts, they choose their own subjects according to the path that they want to pursue after graduation. Harry further take Potions, but on the directional instruction of Horace Slughorn, a professor obsessed with tutoring bright students and creating a web of connection with the most powerful wizards. During his first Potions lesson, Harry is given and old book to practice the day’s lesson and it turns out to be a highly commented book, which belonged to the Half Blood Prince. Following the directions in the book allowed Harry to create a wonderful potion and win a bottle of Felix Felicis- a potion designed to give luck to the one that consumes it. Too much of Hermione’s anger, Harry follows the comments on the book and outshines her in Potions, in the following lessons. Another development of the year is that finally Snape receives the post of Professor of the Dark Arts, alongside with Harry receiving regular lessons with Dumbledore.
Dumbledore gives Harry private lessons in the hope of finding out more about Voldermort’s past and understand how he became obsessed with power. They also find out that in Voldermort’s pursuit of eternal life he had split his soul in seven Horcruxes (objects designed to encapsulate one’s piece of soul). Two of the Horcruxes were destroyed – the Riddle Diary, which allowed the entrance to the Chamber of Secrets, and a ring, of the heirloom of Slythering House. The ring was destroyed by Dumbledore and left him with a slowly progressing dark spell that will ultimately kill him.
The quest to discover the past and understand how Voldermort can be destroyed progresses with the help of Dumbledore’s memories of young Riddle. Harry finds out more about the boy and young man that will become the greatest dark wizard of all time. He is both amazed and shock to find out some similarities between him and the Dark Lord, while also finding out the one important thing that distinguishes him from Voldermort: his power to love and maintain a true friendship with Ron and Hermione. Ron and Hermione help him make sense of all that Harry finds out during the private discussions with Dumbledore, though they don’t agree with Harry’s intensive use of the Half Blood Prince’s Potion Book. Furthermore, Ron and Hermione develop feelings for each other, but Ron is jealous of the friendship between Hermione and Krum and he ends up dating Lavander Brown, which totally impedes the friendship between him and Hermione. Luckily Harry maintains contact with both and somehow manages to go through the year and his own love life, mostly in one piece. Harry starts fancying Ginny, but fears Ron’s disapproval, so he keeps it a secret.
Upon following Draco Malfoy into the Dark Alley, Harry is convinced that Draco plans to kill Dumbledore, and after couple of attempts to smuggle dark art objects into the school, he starts using his Marauder’s Map and Invisibility Cloak to follow him through the school. Unfortunately this disturbs his ability to recollect a last piece of information vital to finding out how Voldermort can be destroyed. Pressed by time and Dumbledore, one night Harry drinks a portion of his Felix potion and manages to retrieve a vital memory from Slughorn. With this piece of information he and Dumbledore manage to deduce the number and the identity of the last Horcruxes.
In one night, almost at the end of term, Dumbledore asks Harry to accompany him in the journey to destroy one of the Horcruxes (the Slytherin locket). They go to one remote place where the young Riddle was on holiday and there they manage to pass all the charms, only to find out that the locket was a replica, the real one being in the possession of R.A.B. When they return to Hogwarts, they find the school being attacked by Death Eaters, and in the feath of the Battle, they go up in the Astronomy Tower, where cornered by Draco, Bellatrix, and other Death Eathers, Dumbledore is being attacked. Harry cannot defend his mentor, being immobilized by a spell under his Invisibility Cloak. Then his worst fears come to life: Draco tries to kill Dumbledore, revealing a pact made with Voldermort, but is unable to kill and so Snape kills Dumbledore. Paralysed with pain, Harry tries to scream and fight back, but only when Dumbledore dies the spell is lifted and he can pursue the attackers. The whole school is fighting the Death Eaters, and after Snape and the others escape the premises the whole school turns to Dumbledore’s dead body and mourns the loss of one of the greatest wizards of their time.
In the aftermath of the fight, Ron and Hermione forget their dispute and rekindle their friendship and also Harry enters a relationship with Ginny, having Ron’s blessing. Without major events, the school-year ends. The friends decide to continue Dumbledore’s quest, to find and destroy the remaining Horcruxes.
What I enjoyed the most in this book is finding the characters developing insights into their own personal life and personal development. You can feel that the characters are growing-up, as they become more aware of others and their feelings alongside with discovering a purpose for their own abilities, and how to use them in their fight against evil.