Line spacing is the amount of space between lines of text in a paragraph. If there’s not enough space between lines, text becomes difficult to read. Place too much space between lines, and the lines in the paragraph don’t look like they belong together.
The line spacing for paragraphs should be 120-145% of the font size. If you have a 10 point font for your book’s body text, the line spacing should be 12-14.5 points. I recommend starting with the lower end of the scale, 120% or 125% of the font size, and see how that looks. As a point of reference, the line spacing for paragraphs on this blog is 130% of the font size.