Don't use an anonymous email. Just email them from an account you use and see what turns up, if it is such a frequent question the author in this day an age could post an errata to their website. They might even have a reworked bit of prose for a second edition or some-related-next-project. The only people I wouldn't email are the people that clearly don't want to be found from a simple search.

It's just an email, don't overthink it. Don't write a rebuttal essay, just express what you are reading and why it doesn't line up for you and send it along. Maybe there's something you're missing or maybe this is a bit of manuscript that was revised a few times and now doesn't line up perfectly. It's no different than going to a friend and asking if they can make heads or tails from it. They wrote a book to share something they found interesting people asking questions and clarifications is what I would think to be one of the few true rewards.