I think to a certain extent, you’ll need to illustrate what’s taking place in the text. Otherwise, it risks coming across as a purposeless, decorative interruption.
A possible approach might be to take a very generalized approach that communicates the setting and emotional qualities of the chapter in which the illustration resides without getting too specific about the illustration simply being a drawing of an event in the text.
Just for the sake of an example, let’s say a chapter was about a gangster heading into the mountains in the evening to bury a woman’s body next to a creek. A literal interpretation of that would be to draw a scene of exactly that — a guy digging a hole next to a creek in the mountains with a body lying nearby.
Or you could step back from the literal nature of that and, instead, illustrate a scene designed more to capture the emotional quality of what the chapter is about. This might consist of a dark, ominous, moonlit night in the woods with dark, jagged shadows, misshapen trees and inky black mountains. You could also pick up on minor details suggested in the chapter and include them, like footprints leading into the wood, a torn sweater left on the trail or the distant beam of a flashlight. Doing it this way wouldn’t amount to a literal depiction of the story, but would be more suggestive in a way that created a visual mystery for a person just thumbing through the book.