WARNINGS - SPOILERS AHEAD. THIS REVIEW GIVES AWAY THE PLOT OF THE BOOK. READ ONLY IF YOU ARE OK WITH THAT.
Harry Potter and the Half-Blood Prince is a lot better than its predecessor -'Order of the Phoenix'. It is dark without being excessively melodramatic(Umbride making Harry write imposition with his own blood) and without begging for pity towards Harry(no one trusting him).
However it is not as good as the first four books were. To be fair to Rowling, when you are keeping most of the characters, the basic storyline and the structure same(Start at Privet Drive, then go to the Weasley residence, and then spend the remaining time at Hogwart's), it is difficult to keep enthralling the audience for 6 books. Rowling seems to have realised this fact judging by the book's ending which gives a peak into the concluding part of the series.
As I wrote earlier, the book fills you with a sense of deja vu until Page 450 or so. Prof Slughorn reminds you of Gilderoy Lockhart. The Half-Blood Prince's textbook reminds you of Tom Riddle's diary. The romantic goings-on are very much like those in Goblet of Fire. Even the Quidditch matches seem absolutely predictable.
But post-450, the book picks up. As a friend and I were discussing last week, the major flaw with OOTP was that it did not take the story forward. It was an extra-dark book which seemed more like a Greek tragedy.
In HBP, the story does move ahead. Dumbledore and harry together go through a lot of old memories of people associated with Voldemort to try and figure out something. They discover that killing him is even more difficult than first imagined. He has used something called divided his soul into 7 parts, and hidden them in 7 different places. He can't be killed unless all the 7 parts are destroyed.
Two parts have already been destroyed, and a third is destroyed during the course of the book. Four parts remain for the concluding book.
Oh yes, the character which is killed is Dumbledore. However I think there is more to it than meets the eye. I suspect that Dumbledore will make a Gandalf-ish return.
The reasons for saying this is that he is killed by Snape. Snape, at the beginning of the book itself, is shown to have meetings with Death-eaters where he takes the Unbreakable Vow that he will do what Draco Malfoy has been asked to do, if Draco fails. This task, as we learn later, is to kill Dumbledore. An Unbreakable Vow means that if Snape breaks the vow, he will die. yet Snape takes the vow.
Now throughout all the books, I have seen a trend. Harry often jumps to conclusions. And Dumbledore is always right. So throughout the book, Dumbledore keeps insisting that he trusts Snape despite Harry's evidence to the contrary. If Dumbledore is so vehement, then in Dumbledore we trust.
I am sure that there will be some catch. Snape and Dumbledore must have done this together so that Snape can infiltrate the Death Eaters and earn Voldemort's full confidence. There will be some way Dumbledore will be able to come back to life in the next book. Maybe he himself has a horcrux stashed away somewhere.
At the end of the book, we are told that Hogwarts will be closed for the next year. This means that the action in the last book will not follow the typical pattern. All romantic problems have been sorted out in this book itself so another school year isn't needed. Ron and Hermione are an item. Harry and Ginny are too, though in the end Harry ends things with her in a manner almost identical to Peter Parker and Mary Jane in Spiderman 2.
The stage is set for a thrilling last book in which Harry will destroy the horcruxes, one by one, and in the end might get help from a rejuvenated Dumbledore, a sympathetic Snape, and possibly a reformed Draco Malfoy (again, there are hints of that).
We'll find out if I'm right in another two years.