Any discussion on postmodernism that constrains itself to one genre, such as typography, is undeniably conservative and consequently could not have been postmodern to begin with.

My tentative suggestion is to take more general theories of postmodernism, throw them into a metaphorical blender along with the history of typographic rules, and take a close look at whatever spills out.

Thanks guys, i have radical Modernism sitting infront of me!

Although it may be true that for something to claim it is post-modern it is by definition, no longer post-modern—you might want to check out Rick Poynor's "No more Rules: Graphic Design and Post Modernism". It is the most recent and comprehensive survey of how post-modernism and graphic design (Including typography) relate to each other.