We used to call them letters The path from the age of the letter to the age of the one-paragraph, stream-of-consciousness email may be a fairly easy-to-follow

We used to call them letters The path from the age of the letter to the age of the one-paragraph, stream-of-consciousness email may be a fairly easy-to-follow one, given the obvious way that technology first enabled, then gradually enforced a vastly lower expected interval between missives, but I'm still curious to see a critical investigation of this process in a McLuhan-esque sort of context. Anyone have any recommended reading?