Font sizes should be defined in em or % (percentage), not by or pixel (px) units or by name, for example, small and large.
The main text of a book should either not have a defined font-size or should have a font-size of 1em. This will ensure ideal readability and font scaling.
When specifying line-height, set the value to a unit-less multiple of the font-size, for example, line-height: 1.2;. Including units, such as em or %, can cause lines of text to overlap if allowed to cascade to an element with a font-size that is larger than the element on which the line-height was declared.
Make sure that fonts included within the content adhere to accessibility guidelines. See Accessibility for information.