JK ROWLING promised that this latest installment, Harry Potter And The Half-Blood Prince, would be pivotal in the series of seven books, and she doesn't disappoint.
After the riddles and mysteries which have been set before fans in the previous five books, readers finally get some answers.
The book begins by showing a dramatically altered wizard world - there have been big changes at the Ministry for Magic, and Voldemort is again powerful enough to come out into the open, surrounded by his followers.
It's a return to a world we've only heard described before - deaths, fear, mistrust, which is even affecting the Muggle, or non-magical world.
We're introduced to terrible new elements of the dark arts, including Inferi, corpses bewitched to do a dark wizard's bidding, as well as the evil secret which enabled Voldemort to survive when the curse designed to kill Harry rebounded.
Yet the atmosphere isn't as utterly dark as Harry Potter book 5 - war may finally have arrived, but it's evident that Harry Potter has been telling the truth, and his relationship with Dumbledore deepens.
And unlike the poisonous atmosphere at Hogwarts in Harry Potter and the Order of the Phoenix, Harry's time at school is more enjoyable - at least partly because of the mysterious half-blood Prince of the title, although when that mystery is finally solved, it's overshadowed by other events.
Romance is on the horizon again - although unsurprisingly not with Cho - helped along by Harry's new reputation as the 'Chosen One'. Ron finally discovers girls in this book, providing plenty of comic moments. Inevitably the course of teenage love never runs smooth.
In 'Harry Potter And The Half-Blood Prince', some of the major characters take a backseat while others bloom on centre stage - especially new character Horace Slughorn, a new teacher at Hogwarts, who manages to be a mix of charm and superficiality, weak good intentions and a touch of genuine wizard skills and powers.
Draco Malfoy evolves, no longer the two-dimensional figure he's been in previous books, he's now more than just Harry Potter's nemesis at school...
Most interestingly, Snape steps into the spotlight - always a shadowy figure, he continues to walk a tightrope, convincing each side he is committed to them.
Both Dumbledore and Voldemort trust him - but he can only be loyal to one. For much of the book, the reader is left in the dark to decide for themselves, but by the end of the book this mystery is solved for good.
As JK Rowling has revealed, there will be more deaths in the final books, including major characters, and by the end of Harry Potter and the Half-Blood Prince, Harry Potter is dealt a devastating blow when he loses yet another person who is dear to him.
It's shorter than Harry Potter book 5, but JK Rowling packs the pages with information and a lot of the book is dedicated to setting the scene for the final showdown in book seven.
Less action-packed than the earlier books, the book seems more concerned more with explanation than events, although as usual the pace quickens to the breathless finale.
Ending on a darker note, all certainties are swept away by the last page, and the final chapter of Harry's story could prove to be very different.
For fans Harry Potter book 7 will be hard to wait for, especially knowing it's the finale, but in the meantime, Harry Potter and the Half-Blood Prince is proving as magical as ever.