As an example, try this book from the MobileRead library:
https://www.mobileread.com/forums/showthread.php?t=97675.
Compare with the version attached, which has two lines added to the top of the stylesheet, setting the font size to medium at the paragraph level and to 75% at the body level.
With the attached version the line spacing is still adjustable, but the lines are much closer at the minimum slider setting.