“Serious” books for adults and young adults do not have illustrations these days. Pictures are relegated to comic books and graphic novels. During the 19th century, however, illustrations were quite common. Pick up early editions of Dickens works or Robinson Crusoe and you will find great line drawings. By the 20th century the pictures had all but disappeared from adult books and by the mid-1950s were gone from young adult (juveniles) books as well. Some of this trend may have been created by editorial philosophy and some by economics, but I suspect the first reason was used to justify the second. The economics of why publishers don’t illustrate are easily understood: pictures are expensive to produce.
In any case, I am trying to bring back illustrations. Unlike most modern science fiction, all of my books and stories have interior line drawings, appendices, maps, diagrams and so forth. I have employed a couple of fine artists over the last five years. Joe Hardwick of Lexington, Kentucky illustrated my first two books while Dheeraj Verma of New Delhi, India has been illustrating my current work. Both do excellent work, although they have distinctly different styles.
I recall enjoying the illustrations of Clifford Geary in the early Heinlein juveniles. They lent a great deal to the story without robbing the reader of the privilege of imagining. Unfortunately, none of the paperback versions of his books have the pictures. To find them, you need to go to Abebooks or some other source of used books and get the original hardbacks, usually discarded library editions. I vote for illustrations! Let me know if you agree.