Think about the great Sue Grafton. Her Kinsey Millhone mysteries are branded with a large single letter befitting a series that began with A Is For Alibi. Small changes in variations of shading on the letters, but overall the typescript is similar. Ditto are the backgrounds similar. Bold colors with no images hidden behind under the letters. No one can pass a Sue Grafton book and wonder who wrote it or if it is part of her series.
The lower one for Uncharted Territory doesn't look like it's part of a series, other than it picks up the red lips from the first book in the red lipstick shotgun shell. Again, the shotgun warns of danger.
My publisher is excellent when it comes to working with his authors on cover designs. We tried to keep the black band in the middle with the shotgun and shell in the loser third for Uncharted Territory, but we couldn't find an evocative image for the top third. Since the book takes place in post-Katrina Mississippi, we tried images of desolation, actual photos from after the tidal surge, an Air Stream trailer, a fancy RV. All are part of the story, but the images didn't work.
With book three in progress, I wonder what we'll do for the cover. Book three takes place in a hospital. I'm thinking of a hypodermic filled with blood...
Betsy Ashton is the author of Mad Max, Unintended Consequences, and Uncharted Territory, A Mad Max Mystery, now available at Amazon and Barnes and Noble.