I don't know anything about your book. You wrote it, you have it published, people liked it. That's positive and awesome!
But situation you described, apply to many if not all, scenarios in daily living. There will always be those who don't just think they can do better, but are convinced they can.
Kind of fanatical "...the way, the truth, the life" crap.
Maybe you could gather all those e-mails from "that guy", share your observations, make some fun, and so on... and write a comic book, and send it to him LOL
Anyways, thanks for sharing. I did giggle at some parts