Astronomicon: Those Left Behind. I hope you'll all agree that this is one of the best designs for an Astronomicon cover. It follows the same basic template as the other three books in the Astronomicon series, but features just about the most eye-catching imagery so far.
In case you are wondering, the image shows a planet being bombarded from orbit. When invading another world it makes sense to take out as much of their defence systems as you can before even considering visiting the surface, and a basic orbital bombardment is usually the best way to achieve that.
This image shows the Eridani homeworld, shortly before the Cephali ground assault commences.