The double-space after the period is good when using a monospaced typewriter, because for readability there should be more space between sentences than between

The double-space after the period is good when using a monospaced typewriter, because for readability there should be more space between sentences than between words (not twice as much, but that's the best you can do with a typewriter unless it has a half-space.) Now that most writing is done on word processors that use proportional fonts and manage the spacing themselves, it's no longer called for.