One space or two?
Here’s a wordy case for never using two spaces between sentences (you can skim it):
Note that buried in all that hyperbole the author says two spaces are acceptable in draft manuscripts. That's the way I learned to do it with all my writing way back in high school English composition class.
But how many spaces between sentences will offer the clearest readability, which is the only important consideration?
Today, every published book has justified type, which means the spaces between individual words are often wider (and sometimes a good bit wider) than the normal word spacing you have in ragged-right composition—this in order to fit a comfortable number of words on each line with minimal hyphenation. Under those conditions, I think using two spaces between sentences stands out a bit more, clearly indicating the end of one sentence and the beginning of the next.
Therefore I’ll continue using the traditional two spaces between sentences. Sue me.
Got 93,560 words on my new novel. Can’t wait to see how it ends.