I approached this post by searching for books designers who have plenty of book cover designs under their belt. I came across John Gall who has designed a ton of books, including the covers for author Haruki Murakami's books.
While looking at these particular designs, I noticed that Gall kept the use of circles in the design of each of Murakami's book covers. Naturally, I assumed this was his way of incorporating consistency among each cover. However, as I looked through his other works, circles were present on most of his cover designs. He either has a thing for circles, or that is his way to make all of his work distinguishable from other book designers.