Orphaned as a baby and subsequently raised (in the loosest possible terms) by his Uncle Vernon and Aunt Petunia, Harry's early years were thoroughly miserable. Although he had been told his parents had been killed in a car crash, in reality they were murdered by an evil wizard called Voldemort. Harry, however, somehow survived this attack and Voldemort subsequently disappeared for many years. Tracked down by an apparent giant called Hagrid on his eleventh birthday, Harry discovered he was a wizard and has since been attending Hogwarts - a very prestigious school for training young witches and wizards. The school's headmaster Albus Dumbledore, considered by many to be the greatest wizard of modern times, has become something of a role model to Harry. His time has Hogwarts has provided also him with some of the happiest moments of his life : he has made friends for the first time (Ron Weasley, Hermione Granger and Hagrid being the most notable) and has become the Seeker on Gryffindor's Quidditch team. (Gryffindor is Harry's house, while Quidditch is the most popular wizard sport). Unfortunately, it has also become apparent that Voldemort's disappearance was only temporary, and his desire to kill Harry has not lessened. Furthermore, some of his key followers are also at Hogwarts. Professor Snape, the Potions teacher who has picked on Harry from the day he arrived, was a noted Death Eater. Draco Malfoy, Harry's arch-nemesis among the student body, is the son of another noted Death Eater...who, thanks to Harry, is now in prison.
The Order of the Phoenix is a secret society formed by Dumbledore to lead the fight against Voldemort and his Death Eaters. Snape, despite being a former Death Eater, has also been admitted - although Harry believes this is a mistake, Dumbledore is convinced Snape has fully turned his back on his former master. However, as "The Half-Blood Prince" opens it appears that Harry's fears may be correct. Snape meets with Draco Malfoy's mother, Narcissa, and aunt, Bellatrix - both Death Eaters - in a small and dirty house in London. Although he may be working undercover for Dumbledore, the details of this meeting suggest he may actually be working undercover for Voldemort. It also becomes apparent that Voldemort, angry that Lucius failed in his mission at the end of "The Order of the Phoenix", has given Draco a very dangerous assignment.
Harry's ambition after leaving school is to be an Auror and, to be selected for this position, he needs to have done well at NEWT Level in Defence Against the Dark Arts and Potions. While Harry has always excelled in Defence Against the Dark Arts, Snape has made it clear he will not accept anyone into his NEWT-level Potions class who hasn't achieved the top grade at OWL Level. When Harry's OWL results come through, it seems his dream is over : while he's done well in Potions, he hasn't achieved the top grade. However, on returning to Hogwarts, he finds that Snape has been appointed the Defence Against the Dark Arts teacher while the new Potions teacher, Horace Slughorn is more than happy to accept him into the Potions class. Since he hadn't bought any books for the Potions course in Diagon Alley, Slughorn lends Harry an old copy of the textbook from the classroom's store. It puts Harry at an immediate advantage over his classmates, thanks to some very helpful notes made by its previous owner...someone who called himself the 'Half-Blood Prince'.
Dumbledore has decided to give Harry some additional 'classes' this year, where he learns more about Voldemort's past. However, they're not very regular as Dumbledore seems to be spending a lot of time away from Hogwarts. Furthermore, Dumbledore seems to have had a difficult summer as he arrives back at school with a very badly burned hand. He also has in his possession a ring that once belonged to Salazar Slytherin and his heir - Lord Voldemort.
With "The Half-Blood Prince", J.K Rowling has written another excellent book - although quite dark in places, it's very enjoyable and is very easily read. I was particularly looking forward to seeing how Harry's rivalry with Malfoy would intensify and I wasn't let down. What did surprise me, though, was how Draco coped with his task - and I'm now very curious to see what he'll get up to in the next book. (The same goes for Snape - for some reason, I'm not entirely sure he's all he appears to be - and I have a feeling that Mundungus Fletcher's brief appearance in this book might yet prove significant). The only disappointment was that Luna and Neville didn't feature more. If you're new to Harry Potter, I would recommend reading the series in order - this is the sixth book in the series and it picks up on certain events from the previous instalments. However, that shouldn't be too much of a burden, as the previous books are also very enjoyable !