In book design, the general rule is that all caps is fine for one line or occasional headings. But within the actual text, it’s best used sparingly.
Why Not Use All Caps?
All caps are harder to read than normal lowercase text because our eyes are drawn to pattern recognition. All caps produces a rectangular box, and we have to really focus to make out individual letters. Lowercase letters are varied—some are tall (dhkl), some are short (aeoru), some have descenders (pgyjq). Mixing them up creates a contour that helps our brain to recognize words. We’re used to reading upper- and lowercase words. Think about street signs. Those that are in upper and lowercase are easier to read from a distance.
All-caps paragraphs are worse. If you want your readers to pay attention to something, you don’t want them to skim or read past the paragraph. It's too much work: Our eyes glaze over and grow tired. It's lazy typography, as the following paragraph shows:
DON’T CAPITALIZE WHOLE PARAGRAPHS. MANY WRITERS BELIEVE THAT CAPITALIZING COMMUNICATES AUTHORITY AND IMPORTANCE. IN REALITY, PARAGRAPHS SET IN ALL CAPS ARE DIFFICULT TO READ. READERS GROW TIRED OF READING ALL CAPS AND SKIM AND SKIP ENTIRE SECTIONS. WHAT DO YOU THINK? IS THIS A FUN PARAGRAPH TO READ? HAVE YOU PERSEVERED, EVEN THOUGH IT WASN’T ENJOYABLE? PROBABLY NOT. BE KIND TO YOUR AUDIENCE AND STOP USING ALL CAPS FOR PARAGRAPHS.
That doesn’t mean you should never use all caps. They work well for short headers, headlines, running heads, captions, and some labels. But if you must emphasize a paragraph, change it up. Use borders, boxes, rules, or color. Add a heading to set it off, or use a larger point size or a contrasting font. Designers have many options to create drama and hierarchy in book design. Use them!