Take a single paragraph from the manuscript and drop it onto a book page. Don’t try to make it nice yet; just set your body typeface, set your trim size, give your page reasonable margins. Read the paragraph aloud, letting your eyes travel from line to line. If you lose your place easily, skim over chunks of text, or if the lines feel like they stretch across the page in a way that causes eye strain, the page is already speaking to you about design. This happens before you even put a line rule on the page or decorate a heading.
Check line length first. Too long, and the eye runs over too far a distance before looping back to the first character on the next line. Too short, and the text looks jittery, and you get too many hyphens. In the interior of a book, you control line length with trim size, margins, gutter, and the width of the text frame. Changing the page space improves the book even if you keep the font choice unchanged.
Check your leading. If leading feels too tight, the body type looks dense; it can feel like a block of text that is too dark. That is particularly noticeable in long paragraphs. If it is too loose, the paragraph breaks apart; each line floats away from its neighbors. You do not want to choose the largest or the smallest setting, just the setting that lets the eye travel smoothly from the top to the bottom without effort. Test this by reading three, or even more, paragraphs back-to-back rather than just one example of “good looking” type.
Check paragraph rhythm. See how blocks of type repeat on a page. Look at the space between paragraphs, the first line indent on the paragraph, the paragraph indentation versus the heading font. You don’t want every paragraph starting with a new visual style; this makes pages feel chaotic. You also don’t want to pack paragraphs so close together that the eye can’t find the pause to signal a change in thought. A few simple rules keep pages stable.
Try two versions of the same spread. The first, the default version, is whatever copy landed on that page. The second changes only line length, leading, and paragraph space. Do not change the font, add images, or redesign the heading yet. Generate both to PDF and do a side-by-side comparison of the two files. The winning version might not look dramatically different, but it should feel easy to jump onto and less tiring to follow.
Finally, check for side effects. Increasing margin space might help with line length, but does that extra space cost extra pages? Increasing leading might help with clarity, but does that create bad break points or a widow or orphan at the bottom of the page? Increasing paragraph spacing might help clarity, but does that make your first page of chapter look disconnected from the paragraph on that page? Book layout design is filled with these kinds of tradeoffs, and every improvement must be seen inside the context of the whole page spread.
Another useful cue is the gray texture on the page. Zoom out or step away a few feet so words don’t resolve, and you see the general texture of the paragraph block. You want the body type block to look evenly gray, without being too heavy and solid, or too scattered and broken. When you get line length, leading, and paragraph rhythm right, most readers won’t notice you changed anything about the design. And this quietness is exactly when you know that you’ve done your job.