I'm the author of The Broken Empire, The Red Queen's War, & Book of the Ancestor trilogies - spare ideas land here.
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These are the ones I have which are different from my physical book covers (aside from the covers you have listed).By Peter BrettThe Warded ManThe Desert SpearBy Jim Butcher;Fool MoonStorm FrontGrave PerilSummer KnightBy Michael J SullivanThe Riyria Revelations series1st 2 books in The Riyria Chronicles- The Crown TowerThe Rose and the ThornBy Django Wexler3 out of 5 of The Shadow Campaign seriesBy Brandon SandersonThe 1st Mistborn trilogyBy Robin Hobb (not sure about this one because there are so many different covers for her books)The Tawney Man trilogy
What's the rationale behind the difference between the covers? Different audiences, different publishers or just for the sake of it?
It may be a different publisher with a different budget for art, and/or a different view of what their market will accept in terms of a cover.
The evolution of the art between the Broken Empire books is so good though. With these the font changes the tone of the art is completely different which makes it feel less like a series.