Although I'm not sure it covers this specific issue, History of Graphic Design by Phillip Meggs is a standard and could be a starting point.

Flush left/rag right would not be any more difficult than full justification whether in a manuscript or a printed document. I would imagine the flush right/ragged left came about with the invention of movable type. Lines set with movable type on a press bed can easily (relatively) be moved from left to right justification whereas a calligrapher would have had a difficult time knowing where to begin the lines of handwriting in order to end them at the same point.