Eragon is set in an entirely new world, with its own created history, politics, and even geography. While we may recognize elements of this world in our own, we're dealing with fantasy here, folks. Frankly, nobody who lives in Alagaësia has ever read Shakespeare. Or Milton. Or, heck, even J.R.R. Tolkien. So, there are no real shout-outs to speak of, since the characters' references are all either to invented books or invented histories.