Cy Twombly: a monograph. Paris: Flammarion, 2005.
My sense at the moment is that this book is a very good place to start with Twombly. (Best website here).
And this, from John Berger:
"I know of no other visual Western artist who has created an oeuvre that visualizes with living colors the silent space that exists between and around words. Cy Twombly is the painterly master of verbal silence." READ THE ESSAY
And, if you happen to be in London: Twombly and Poussin: Arcadian Painters (thru Sept 25) - where you can see Tacita Dean's 2011 film on Twombly, "Edwin Parker." AND see the set of videos from the exhibition with narration at the Dulwich Gallery multimedia channel!