I've now written seven books, at least three of which were category bestsellers on Amazon. They all get really good reviews and are legitimate enough that foreign publishers have bought the rights to two of them and that domestic publishers have tried to offer me a book deal.
For many people writing a book is a bucket list item, which seems a little bit funny to me because it's actually a relatively easy thing to do. You can write a book in approximately two weeks, plus some time for editing and publishing. My first book (and, admittedly, my worst) was written in two days and was decent enough that many people emailed me telling me it changed their lives.
One of the biggest things that seems to get in peoples' ways is that they believe that writing a book is some huge daunting task, and that the book must be perfect. If you think that way, you'll trip over yourself and psych yourself out and never actually finish the book.
The first thing to realize is that the point of writing a book is to share information with people. If they receive and understand the information, you have succeeded. Take your ego out of it. Your book doesn't have to be fancy or make you seem like a scholar, it just has to help people (or entertain people).
You know that feeling when you're sitting across from someone and they're prattling on about something in which you have no interest? They aren't actually trying to bore you, they just don't know any better. Which begs the question—are you ever that person?
In reality I'm sure we all bore someone sometimes, but we can work on reducing or eliminating that to make sure that it happens as infrequently as possible.
First, think about what benefit the information you're about to share has to the listener. Will they be entertained? Will they learn something useful? Are they a good friend who will want to share your joy or help you with your problem? If there's no benefit, don't share the information. Save it for someone else.
A prime example for me is politics. During the election everyone wanted to talk about politics, which was never an enjoyable experience for me. I was forced into tons of conversations, very few of which were positive experiences.