Writing a book is on my list of things to do in my life. It's a busy time in my life, and I'm not 100 percent sure I possess the skill or knowledge to write adequately about the topic at hand, but it's surely an interesting idea, and one which I will pursue.
I'm still wrestling with doubts about my level of expertise after writing umpteen computer books. If I waited until I felt "100 percent sure" I was as proficient in a subject as one of the technical authors I admire, such as Elliotte Rusty Harold or Matt Neuburg, I'd still be waiting.
I think the best rule for dealing with self-doubt was offered by William Strunk Jr., as described in E.B. White's introduction to The Elements of Style:
He scorned the vague, the tame, the colorless, the irresolute. He felt it was worse to be irresolute than to be wrong. I remember a day in class when he leaned far forward, in his characteristic pose -- the pose of a man about to impart a secret -- and croaked, "If you don't know how to pronounce a word, say it loud! If you don't know how to pronounce a word, say it loud!" This comical piece of advice struck me as sound at the time, and I still respect it. Why compound ignorance with inaudibility? Why run and hide?