The author says:
A Fifth Magic is a dark fantasy novel set in a fractured post-civil-war society where magic is both feared and tightly controlled. Detective Michael Hawthorne enlists the help of sceptical academic Laura Blythe to investigate a locked-room murder mystery that mirrors the methods of a war criminal they believed to be long dead.
Nathan says:
There’s one big problem: It’s too murky. There’s not enough color or contrast to catch the eye of a browser who sees the cover in thumbnail in a string of other cover thumbnails.
Fortunately, fixing that is super-easy — barely an inconvenience! Thirty seconds of playing with levels and saturation gave me this:
It’s not anything like a final version, but it gives you an idea of the possibilities once you look at the artwork and say, “How can I make this more arresting?”
Other comments?
Not much to add to what Nathan has said: increase the contrast and saturation in the artwork.
Indeed, this is another cover where little more than our esteemed host’s recommended minor tweaks are needed. Other than those, the title and byline are large enough to be legible in the thumbnail, but could be even a little bigger. Optionally, you could expand them just enough that they won’t get cut off if the cover’s margins get trimmed on a physical copy.