Some suggestions on how to gain confidence in front of a crowd:
1. It's NOT about YOU, it's about THEM. In other words, they came to hear about your book. You know more about it than they do, so put yourself in their head and try to speak to their need. (In this case, their need is to know more about you.)
2. Whether it's a large crowd, or only a few people, make your talk personal. Talk to them as if you're talking to just one person. It can either be a person who seems to be looking at you attentively, or it can be a friend who's not there, but is in your mind's eye.
3. Just be you. No need to be pretentious. Have an idea of what you want to say, but don't read them a speech. Instead, speak from your heart and in your own words. Think of it as a conversation.
4. It's a two-way street. Once you've said what you wanted to say, then take questions. That way, you'll know what THEY (remember, it's about THEM, not you) are interested in. Keep your answers short and then ask, "did that help?" That gives the person a chance to ask a follow-up to seek better clarification.
5. You might consider checking to see if there's a Toastmaster's club in your area. It's a great way to get experience as a public speaker, and get helpful feedback on your speaking style.
I'll stop there. I could probably go on. Bottom line: once you realize you actually have something to say, then you can relax and just say it and not worry about what people think about you because you can't control that. All YOU can control is your message and your delivery. You're just talking with someone. You do it every day. Only this time, a few extra people will be eavesdropping.
Final thought: HAVE FUN WITH IT.