Well, I think you can discuss themes and character motivation and so on even with well chosen, high quality easier books. DD did some of this even in 2nd grade when they read a book called Stone Fox. You certainly can do it with a book like Harry POtter. I'm seeing less of it this year at school, for some reason...meh. (More, instead, of the infernal "tricky" reading comp worksheets.)
But as for continuing to build their ability--enh. When you have a child reading this well at this age...it's just going to come naturally. I don't worry about this any more for either of my kids, other than to do my own homework in terms of what might work well for them, and then to strew the path with those options. (Well, this is actually quite a bit of work. But it's not focused on level as much as thinking about what they would like, what's emotionally appropriate, and what is generally in their range.)
