Establishing Credibility uses the ideas of ethos to outline the importance and how to establish your credibility as a speaker. The author goes on to speak on the importance of credibility as it is the single most important thing that a speaker can have. To be credible adds weight to your argument, your points and words, thus really solidifying your points and being overall, more persuasive.
I think the most interesting idea that the author offers is the idea that good character or solid credibility may only be an appearance. I think this idea it something that was always sort of apparent to me but, I never really considered when thinking about the credibility of speakers.