This information is for users designing their book in a page layout program like InDesign or any other program that lets you adjust the weight (thickness) of lines, or the CMYK values of individual lines, text, or background colors.
Minimum font size
In general, use a font size of at least 10 points. If your text is smaller than 10 points it may not print well.
Using a Light typeface? Go even larger. If using one of the Light typefaces you must use a minimum of 12 points.
Opacity: if you've adjusted the opacity of the text (to make it more transparent) then you should also use a larger font. There's no magic formula for this but a good starting point is at least 12 points.
Line weight
We recommend a minimum line weight of .25 points.
We do not recommend using rich black (a mix of CMYK values) for lines. Use K only, with a minimum of 30K. (See Minimum ink coverage)
Minimum ink coverage
The ink coverage for any one ink value should be no less than 15%
The minimum total area coverage (TAC) should be no less than 30%
Using less than 15% for one color, or less than 30% TAC, might result in that content not printing as expected.