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.
justification
Widows, Orphans, and Other Abominations in Book Design
By Bryn Kristi
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.