As you know, I have a few books on self-publishing. The first was originally published in 2009. I designed the original covers myself, and felt they worked for nonfiction: The main point was that the titles were easy to read in the thumbnails.
The big problem for me was that my covers didn’t have a unified look. So I hired Melissa Stevens (www.theillustratedauthor.net) to make them more unified and to add an image that might help them pop. We settled on a geometric approach, arranging the covers of my books in a cube, spheres, and a pyramid.
She also designed a matching header (you can see it now at WordPress and Facebook), shaped like a cylinder.
The cover that impressed me most was the boxed set (coming soon) that I used for this cover reveal above. I like this perspective, which shows off the front cover while still allowing for ample detail on the spines.
Chris McMullen, Author of A Detailed Guide to Self-Publishing with Amazon and Other Online Booksellers