back in the old typewriter days, students were
instructed to put two spaces after a sentence.
ever since wordprocessing, though, some people have said two spaces are no longer required, that it is an unnecessary leftover from earlier times.
those people were wrong. if you have one space after a period, sentences run together too much.
but...
the thing is, it's actually a lot easier to edit text if you only have one space after a period... that way, you can do a search for two spaces, and that search should come up totally empty.
thus, to make life easier on the writers out there, your software should create the smidgen of space necessary to separate two sentences sufficiently.
so, if you're making an e-book, use just one space.