The most important thing I learned about book illustration, I learned on C-SPAN of all places.
They had a "book talk" show, in which a professor was talking about N. C. Wyeth -- the greatest illustrator of all time, IMHO. This professor showed a bunch of illustrations of Treasure Island from before Wyeth, and they all had something in common. They all showed what happened. That is, they showed the moment when all the tension was released.
Wyeth, on the other hand, had a different approach. When something exciting was going on, he showed the moment before it happened. The moment when the outcome was still in question, and when the tension was at its height.
This was one of the pictures he showed -- just before the moment of violence. Will Isreal Hands throw his knife? Will Jack Hawkins flinch, or will he shoot?
There were other illustrators who had this sense of drama, of course, but none like N. C. Wyeth. The pure visual beauty of his work is something to behold. The strong diagonals, the use of vivid detail and odd shadows and blur to create psychological effects. The scenes were huge and dramatic like the Romantic painters like David, the lighting, though, was more flat and modern, like the Realists, who tended more toward ordinary subjects. And that brings it right back into the immediacy and drama.
Which takes me back to the dramatic timing which made Wyeth brilliant. It's what turns an illustration from mere reporting to storytelling. It's also what changes a work of art from a landscape or portrait to an event.
Many of those illustrators which that professor used as examples most likely worked for newspapers in their day-to-day work. Until a century ago or less, newspapers relied more on drawings than photographs. "What happened?" was the question they generally worked with, more than "what will happen?" So it makes sense that the emphasis was on reporting rather than the storytelling - at least in those days.
Great photojournalists now, of course, understand the drama of tension. If you look at prize winning news shots, they are like a Wyeth illustration, full of action and drama. They are images which not only inform you of what happened, but make you want to know what happened next.
It used to be that most fiction was illustrated -- in magazines, in newspapers. Often multiple illustrations. These days, though, illustration has pretty much gone by the wayside (other than for children). Most of the time, all we have are the covers to tease and tantalize us.
And if all we have are covers, we have an extra problem in this modern age of ebooks. All you see is an itty bitty thumbnail, and even if you have a beautiful Wyeth-quality cover, the reader is unlikely to be swept up by the drama. They won't be able to see it that well.
However, I don't know if you ever noticed books from the old days, before they had a paper dust cover -- but even though it was the heyday of illustration, the cover itself might only have an impressed logo-like image. A crown or a ship. The really catchy dramatic illustration would be in the frontispiece inside the cover.
In this modern age, even as cover images shrink to postage stamps, and the books themselves are text-only, we can bring back the frontispiece: by putting it on a website.
We have new opportunities now to make illustration a part of the overall online brand or identity of a book or series. The web gives us a wonderful opportunity for supplementation. I hope that people in publishing will make use of it.
(Last minute addition: and golly I just noticed a few days ago that Steve Perry decided to sponsor an Art contest on his website. Exactly the sort of thing I was talking about!)