Readability consists of two elements, language and the other literary factors being the ones most often focused on by authors. But almost equally important is the layout, especially the font used (serif fonts are much easier to read that sans-serif), characters per line and leading, because readers are discouraged by something that confuses the eye or otherwise is difficult to read. Scholarship that is worn lightly combined with a reader-friendly layout can be crucial elements in the success of a book.
This definition is extracted (and expanded on) from the book Getting Published: A Companion for the Humanities and Social Sciences by Gerald Jackson and Marie Lenstrup. It was first referred to in the blog Getting Published in a post on the readability of fonts and diacritical marks.