What really sold me on the first book was the cover by Daniel Dociu. He's well worth a Google image search as he's produced hundreds of superb paintings (you might start here, for instance) and designs for video games. I was intrigued to discover that the front and back covers of the Orbit paperback were canibalised from the same painting.
I posted a breakdown of the first cover as a random scan back in August 2014. Since then I have picked up the second and third books in the series, which continues to use elements of Dociu's artwork for both front and back covers.
The second of these covers is particularly interesting as the posted clean artwork is slightly different to the final artwork used on the book, which has zoomed out slightly; the spacecraft is higher on the horizon; and the upper structure (top right of the original) is far less complex and its yellow panels have disappeared on the final cover.
The third cover also makes some minor changes, with the superstructure at the top right enlarged so that it now obscures more of the background and one of the flying craft.