A striking book cover is what encourages potential readers to stop and pick up (or click) your book. It needs to captivate, entice, basically, it needs to have shelf appeal. When you see it across the room, you want to pick it up, take a closer look, flip open a few pages.
Of course, book design is not just the cover. A beautifully designed interior will make your book a pleasure to read, and to hold, because a physical book isn't just a medium for your writing, it's a piece of art in itself. And everything from the typeface to the margin settings should help tell the story. The right chapter headings set the mood, the appropriate spacing on a page allows your reader to breathe, the right glyphs add character to your pauses.