I, too, find it of immense interest. I recall being taught that
paragraphs were indented 5 spaces. I also remember that this was not
universal, as there was another variation, called "block style" which was
not indented at all.
I have been looking at a number of books, and noticed that some have a
single space between sentences, others two spaces. This occurrs in both
US and UK editions. I even noticed a German grammar in blackletter that
uses the double spaces, although it was US printed. I do notice, though,
that the double spacing enhances the readability of the text.
In a lawsuit I was involved in regarding an IEEE audio serial interface spec, the expert
attempted to explain block/frame
identifiers to a lay jury by analogizing them to punctuation identifying the beginnings of
paragraphs and sentences. An indent
indicated the start of a block (and the first frame in the block) and a period indicated
the start of a frame (that was not the
first frame in the block). Under this analogy, the spaces after the period were
superfluous.