This may be in all the cookbooks and I have simply never come across it, but in making the traditional apple pie, my wife forgot to peel the green and red apples.  We discovered it as she was putting on the top layer of dough.  We decided to let “let’r rip” and it was a fantastic discovery.  First, the skins bled ever so subtly so each piece of apple was slightly colored, light pink or greenish-yellow which was quite attractive, and the skin imparted just the subtlest hint of a slight bite and tartness that contrasted with the overall sweetness of the pie.  It was really great, and I will never peel apples again for an apple pie.  You do want to cut the pieces thinly, rather than into quarters or eighths. Try it!