Charles Geer is a children's book and cover illustrator who did the beautiful drawings for many of the William Morrow editions of Mary Stewart's hardcovers. I've tried to do research on him, but have been unable to find too much. Here are a couple of his Stewart covers:
I came across an announcement for a show of Geer's work at the Mulford Gallery in Rockland, ME. From the announcement, it sounds like there will be drawings from his Stewart covers there, though I'm not sure. Anyone live near Rockland who can go check it out for us? The show runs through June 15.