Double spaces are used in the original edition of Captain Blood at the end of sentences. To duplicate the effect in HTML I put each sentence-ending space inside a <span class="s2"> tag, and add CSS of either .s2 { word-spacing: 0.25em } or .s2 {font-size: 200%; line-height: 0; }. Either method is standard CSS1 and thus works in everything reasonable.
Which means both ways work in iBooks; in ADE and Kindle Mobi word-spacing doesn’t work, and in KF8 and Kindle Mobi any line-height less than 1.2 doesn’t work. (Though in Kindle Mobi increasing the size of a space character doesn’t affect the line’s height on an e-ink reader; it does on the iOS app though.)
Source:
http://extraordinarycommons.com/double-spaces/