I happened to have made the box set cover for this series already, and so I took the look I developed for the box set cover–a spired, futuristic cityscape and lush skyline lights–and translated it into covers for the three books.
This series involves elemental magic, so each cover has an elemental theme, too:
Mark of Four’s is water:
Shadows of Uprising’s is air:
And Guardian of the Vale‘s is fire:
(All images remixed under CC 2.0)