It's a New Era: In the typewriter era, good style required you to put two spaces after a period. Typewriters used "monospaced" fonts, meaning that every letter or glyph took up the same width. With monospaced fonts, two spaces after a period looked better and we're needed to set off a sentences from each other. When we moved into the computer era, more than two decades ago now, most fonts in common use are variable-width. These fonts look better when you use only one space after a period.
Even so, a surprising number of people keep to the habit of inserting two spaces after a period. This either makes more work for your desktop publisher, who will eliminate them, or will leave your book looking a little amateurish if your desktop publisher doesn't better than to eliminate all those extra spaces.--The Publishing Pro, LLC