Cornwell, a contemporary of Rockwell's (Rockwell was a fan, in fact, who sought Cornwell originals for his wall) whose preliminary charcoal drawings -- he seems to have seldom used photography for his prep work -- are collected by illustrators today.
He could draw like a dream.
His early romantic/adventure illustrations are my favorites. Their painting is a little softer, the color subtle. Many are boldly designed vignettes. The women are winsome, the men heroic.
Oddly, his work suffered as he matured.
In his middle period, his sense of color atmosphere abandoned him, and the several Biblical epics he illustrated are oversaturated in Technicolor reds and blues and golds.
He idolized the British artist Frank Brangwyn, and like Brangwyn, became a muralist later in life -- a moneylosing shot at immortality.* But influenced by Brangwyn, his treatment of the human figure, and of spatial design, became increasingly mannered.
He painted angular figures overlapped in strange and flattened clusters, like crowds of dancers photographed with a telephoto lens. The color got stranger, too.
But there never was a better draughtsman, particularly in the drawing of form and drapery, in the history of illustration.
He also epitomized the ethic of good visual research; every buckle and strap and switchcover are depicted with explanatory clarity.
The Cornwell book (Dean Cornwell, Dean of Illustrators, by Patricia Janis Broder) is around, though not at much of a discount ($98 was the lowest I found on the web). Any student of the field should have it.
*And futile. His most famous murals, in the L. A. public library, were lost in a fire about a decade ago. His illustrations are widely scattered, and live on in reproductions.