There are a lot of things that can make a book unreadable: the content, the prose, the plot holes. The list could go on and on. For me, nothing makes a book more difficult to read than badly justified type.
Books come in all shapes and sizes, from picture books for children to 1000-page, text-only novels. No matter the book or who it’s for, design matters. The design can set the tone and expectations for a book. A reader expects something much different in the design of a horror novel compared to a romance. Good design is invisible, especially with text-heavy books. It is the lack of distraction that makes the design good. While image-heavy books can—and should—focus on aesthetics, how they are put together and designed should not distract from the content.