I want to get better at public speaking. I'm taking a couple of approaches. One is to join Toastmasters, which is a freakily American public speaking club, to get the practice in (there's a branch at work). The other is to read up and get some theory.
My boss at the time recommended a video of a talk by Cara Hale Alter, I watched it, and I bought the book. The book's all about how to present in such a way that you achieve, well, credibility. So, it covers standard "how to present" skills, but with an emphasis on those things that make you clear and authoritative. Quite a lot of "stand up straight and speak clearly", to be honest. It's a short book, and not terribly dense.
On the other hand, it does the job - the advice provides nicely concrete things to work on. Videoing yourself is the core to the book's way of improving yourself, and I've been doing that. Learning how I look when presenting is both painfully unpleasant and very useful for learning with!
A younger version of me would be very wary of faking-'til-you-make-it authoritativeness in this way. Current me is happy enough that I'm not much worse at making it up as I go along compared to everyone else, and that my audience deserves to get my ideas explained clearly and in a non-distracting way. I'd happily send my younger self a copy of this book.