I have technical background and founder of software startup. Now, I started learning the importance of Communications Skills in demos and meeting with prospective clients.
Any books or courses recommendations to conquer this field as well?
I second Toastmasters. Speak often.
And even moreso I'd post videos of yourself on Facebook and ask for hard and constructive criticism. Pick a semi-simple topic...how google works... and do a 5 minutes on that. Use a few props.
Self-improvement books are not communication books. Neither are persuasion books...they are great for tips about content but you really want to learn about being both relaxed and present - to be able to speak on or off-the-cuff to group or in-person.
I like Resonate by Duarte in that it speaks to preparation, but doing it with feedback is the direct route. I can speak to that (NO PUN!!!) from experience.
Rather than books and digital learning. As you are a founder of a company you will be surely financially fit. So, why not you appoint a personal lady secretary who is good at speaking English. And you can learn very easily and in best time. :-)