I ask the question because I have acquired the notion that a literary convention rules out illustrations for a 'serious' book. This convention is fairly recent and perhaps not across-the-board. There was a time, not so long ago, when novels intended for adults were illustrated (remember those old Readers' Digest compressed novels?), but that seems to have fallen away in recent decades.
To illustrate the novel I decided to use the pen-and-ink medium for a couple of reasons. It is a technique I was fairly confident with, as opposed to painting which I'm no good at. It would also reproduce well on a black-and-white printed page, and it would hold its quality regardless of the printing quality (greys might have been inaccurately reproduced).
Here are a few examples of how I did an illustration. The first is the newly-crowned emperor Ennodius, on p156 of the printed edition - like several other illustrations it's not in the kindle version, so you do get slightly better value for money if you buy the paperback. I decided to base my style on Robin Jacques, a master of the technique back in the 50's and 60's. He illustrated Steel Magic, a fantasy children's book I won as a prize for a writing competition when I was ten. Here are some examples of his work from that book.
Before starting I needed some raw material. First Ennodius's upper torso with the clothing and armour. for that I used this image (from Wikimedia Commons)
Then the face. This bust of the emperor Elagabalus from the University of Texas' Introduction to Ancient Rome came as near as I wanted.
Using these two I first did a pencil sketch then followed with the ink, first doing the main lines - outline of the head, edges of the cloth - then following with the details - the folds in the cloth, the details of the face the hair, uniform details. Finally the finished result:
Merovec was a bit more difficult. I found this image for his helmet, which I modified.
The face was largely made up. for the hair I needed an example of braiding and found this. OK, not exactly a barbarian warrior but it did the trick.
And the finished result:
For the picture of Aemilia and her father Tarunculus in their apartment I found a couple of willing volunteers to pose. Here is 'Tarunculus'. He is Brother Crispin from the nearby Mariannhill Monastery. I persuaded him to let me take a few snaps of him whilst there for a Christmas retreat.
Then 'Aemilia'. She was a fellow-retreatant.
I found references for the plates, lamps, lampstand, chest and the table. The end result: