Speak Easy is a voluntary program designed to increase diversity in tech conferences through dedicated conference spots, mentoring and events - https://speaking-easy.com/
Scott Berkun - Confessions of a Public Speaker - I remember reading and enjoying this one. https://www.amazon.co.uk/Confessions-Public-Speaker-Scott-Berkun/dp/1449301959
Also, a Ted playlist of talks on the topic - https://www.ted.com/playlists/226/before_public_speaking
What have you found useful?