That starts in 6, but until right at the end it's Dumbledore's task, which he then passes on to Harry for the main plot of 7.

I think it's good that the final book is going to be two films, as I think that's the first one which really justified its length and where editing would have done it a major disservice. Of course, they all gripped me right through when I was reading them, which is more than can be said for plenty of better-regarded writers' shorter books.