Books in 1918 were starting to use real people portraying the characters instead of illustrations, as books had previously done for centuries prior:
The two “illustrating photographers” employ a scout who is sent out to the locations where suitable models for the character required may be found, but most of the new models — and the list of 3,000 is receiving constant increments — come through the good offices of those who have already posed and who spread the word that it is easy money for pleasant work. When a story deals with east side or rural types or some other specialized characters, the photographs do not reproduce made-up actors, but originals — real east side tradesmen, real farmers from the high grass.
The modern-day descendants of those photographic pioneers include Jason Aaron Baca, who has posed as the male model on more than 600 romance novels and counting.
Using the Camera to Illustrate Fiction: Models Pose for Photographs Showing Scenes in the Story — How Two Artists Originate the Plan
From Sunday, January 6, 1918