All credit for the idea goes to /u/ZenAnonymous (https://redd.it/8rm3sv). My post: https://redd.it/9ee646
The covers are 5x7 pixels scaled up. They just use three randomly generated colors (background, foreground, bookmark). The frame of the book is just the background color, but darker. The spine of the book is that color but even darker. The page colors are the same for each book.
They are sometimes mirrored horizontally and/or vertically.
