The order of Phoenix is in full operation, Harry's Godfather - Sirius Black is dead and there is fear everywhere. The ministry is still not ready to accept the return of Lord Voldemort and Albus Dumbledore, the Headmaster of Hogwarts School of Witchcraft and Wizardry has decided to give private lessons to Harry Potter, our Hero.
Quite unexpectedly, there has been a change of charge and Professor Snape gets his favourite subject, 'Defence Against Dark Arts' while a certain Professor Slughorn gets to teach 'Potions'. This change allows Harry to continue 'Potions' in his 6th year, despite not managing the top grade in 'Potions' O.W.L. in the previous year. Here he gets to use a book which previously belonged to someone who called himself 'Half-Blood Prince' and who had written wonderful side notes about spells and potion making. With the help of these side notes, Harry performs well and becomes a favourite of Professor Slughorn.
Alongside, Draco Malfoy, a Slytherin and Harry's biggest rival was upto something. He spent all his time in the Room of Requirement, doing something, keeping his faithful friends, Crabbe and Goyle on guard, wherever he went. Malfoy does not even trust Snape, his most favourite teacher.
Some strange accidents keep happening here and there. Katie Bell was attacked and Ron Weasley gets poisoned.
Meanwhile, Dumbledore has told Harry much about Harry's past and how he had divided his soul into 7 fragments and that he could only be divided if each of these 7 parts (or Horcruxes) were destroyed. He also takes Harry to hunt down one Horcrux , Slytherin's locket. When they return, they find that the Death Eaters have taken over the school. Snape kills Dumbledore and Harry discovers that the locket they had brought was fake.
J.K. Rowling actually has the emotions of the readers in this book. How Harry finally decides that he has to fight his own battle and that he can no longer have other people shield him and die - like his parents, Sirius and now Dumbledore.
A powerfully mesmerizing and power-packed book. What more can be said about Rowling's books, they are brilliant beyond human comprehension. A most definite must read for one and all. Do read the book to find who the Half-blood Prince was and why?