For my humorous poetry book releasing in March 2020, 'A Hatful of Dragons: And More Than 13.8 Billion Other Funny Poems', I have been working on various iterations of the covers for the last year. Here is the current finalist for the front:
And here are some variations for the back. I made several different ones so we could accommodate varying amounts of back-cover text (such as blurbs and reviews). We won't actually know how much space we need for the text till very late in the game, so making lots of variations gives the book designer flexibility:
As you can see there is a ton of work that goes into the making of a book that is invisible to the reader.