See the following for more information:

Line Height

The "Line Height" setting controls the line spacing you want to see when reading eBooks. The default options provided are

The default line height is "Medium". If you are familiar with HTML and CSS, then you should know that any valid "line-height" value can be used here.

Note: This setting is applied at the top level eBook element and acts as a default setting. If lower level elements define line height options, then those will take precedence over this setting.

Default Display Settings
1. Default Display Settings
Set Line Height
2. Set Line Height
Select Line Height Option
3. Select Line Height Option
Line Height Option Applied
4. Line Height Option Applied