When writing your book, stop writing these phrases, “I feel; I believe; In my opinion; To me; etc.”
It just sounds too wishy-washy. You need to instill confidence in your reader. If you’re writing a book on your non-fiction topic, you are the expert. So, when you’re self-editing your book, get rid of these phrases. If you don’t catch yourself doing it, then do a “find” command for these phrases in your Word document.
You are the expert. Before you can write a good book, you have to believe it. Then you have to write so your readers believe it too.