Schoonover is best known for his outdoors North American scenes. I wrote about one of them here.
He painted more subjects than that -- even illustrators as well known as Schoonover need income. One such project was creating illustrations for Edgar Rice Burroughs' "A Princess of Mars," the first in his John Carter of Mars series.
Burroughs is best known for his Tarzan books, and Schoonover's John Carter seems to resemble J. Allen St. John's depictions of Tarzan. I wrote about St. John here and here.