Finished reading Harry Potter and the Half Blood Prince last night.
While the first five books are essentially school stories, which also stand well on their own as individual books, HBP makes a departure from this. HBP takes a leap from these – it is set in Hogwarts, but it is not a school story. It presupposes that you have read the other five (not that it should be otherwise!), and heavily depends on knowledge from them.
Another major departure from the earlier books is the unabashed importance given to the storyline and advancement of the plot. Detail is given the go-bye, and action is the key. By doing all this, JKR has set up a mega finale to happen in the seventh and final book. Rather like Kill Bill, HBP is the first in the series that has no resolution – only a lead in to the next book – and like the two Kill Bill films, books six and seven are part of a set – a set that is incomplete with just one of them. So begins another long wait till the next one comes along!