The story as I have heard it is that once upon a time typewriters used monospaced fonts, which means that every letter gets the same amount of space, whether it is fat or thin. The result was that you could end up with relatively large white spots within a sentence, therefore it was useful to double space a sentence end to make such a break more pronounced. Is this true or not? Who knows. There is a surprisingly large argument on this matter.
I don't think it really matters. We don't, for the most part, type on typewriters anymore, and most typefaces are designed to look good with only a single space after the sentence. I think that most style guides today recommend the single space.
As for e-books, I think you'd have to check with their specific recommendations. Some might allow either depending on author preference. Some might auto-format regardless of what you do. I'm not familiar enough with them to say, but I'd say single spacing is a safe bet.