Harry Potter questions??

I have been reading through the Harry Potter books in the last month or so and I just finished Order of the Phoenix...LOVED it!! Although it seems like most of my questions have been answered thus far...why Harry can see into Voldemorts mind, why Voldemort couldn't kill him...etc. Of course I am going to keep going ...I am bout 50 pages into Half blood prince. But I was just wondering if there is still much to the story that I don't understand or if the next two books are just about killing Voldemort. Oh and please don't give away to much.