I've written before about the importance of typography in book cover design. While a book cover is designed to attract attention, the interior is the opposite. It should be so readable that you don’t even notice the typography. Good editorial design is inconspicuous and allows the author’s words to be communicated directly to the reader without distraction. There should be synergy between what the words say and what the design says. It sounds simple enough, doesn’t it?

There are a multitude of details that must be considered, including your cover design, style of writing, genre, target audience, and page count — just to name a few. The first and most obvious choice to be made is the font. Nonfiction books include multiple styles in levels of information ranging from chapter titles, heads, subheads, and body text. The style, size, and width of the fonts must be chosen to ensure that the reader’s eye goes to the most important thing first. Different fonts take up different amounts of space per line, so the right one may reduce the number of pages in a long book or increase the size of a shorter one.

Old Style serif fonts like Garamond and Caslon are most commonly used in books, as well as the newer and popular Minion. Because they are so familiar and so readable, they don’t “get in the way,” by distracting you from the copy. Sometimes a book like a memoir calls for a more delicate look, or a business book may require a dominant, thicker typeface. Sans serif fonts are often used for heads and subheads, while serif fonts offer a more readable and lighter appearance for body text. For ebooks, delicate fonts with thin lines are less readable, and there are many fonts designed specifically for reading on screens.

Consideration must also be given to the margins of the book. The center margin needs to be wide enough so that the reader doesn’t have to pull the pages wide to read the part in the shadow of the binding. The outside margin is traditionally left wide, so your thumb holding the book doesn’t obscure the text. A good typographer will also ensure that the book is laid out on a grid so that each page begins and ends at the same level on the page. If the lines of words on each side of the paper line up back to back, they are less likely to show through the page and make it hard to see.

Once the margins determine the line length, the font size should also be selected accordingly. For maximum readability, there should be between 62 and 72 characters per line. Fewer than that makes the eye fatigued as it needs to work harder to keep its place when reading from one line to the next. If the text is too large for the line length, the eye has to move back and forth too much for comfort. The space between lines, or leading, also plays into readability and can make it harder or easier to follow the text.

The last thing to be considered is one that is often overlooked by amateur typesetters. This is the way lines end and the sometimes awkward spacing caused by justified type. You want to avoid "widows" and "orphans." A widow is where one or two words end up alone on a line at the end of a paragraph. An orphan is when one line of a paragraph stands alone at the beginning or end of a page. Too many hyphenated words at the end of a line can also be distracting to the reader. Rivers can also appear, which are when spaces between words line up vertically or diagonally on the page, creating a visual white line.

All of these things and more go into the interior typography of a book. Programs like Microsoft Word make these difficult to address, which is why most pros use InDesign for page layout. A professional book designer knows how to lay out the pages of your book so that it suits your style, attracts readers, and keeps them focused on your message. If it is done well, it is indeed invisible to the reader.