This actually raises a very good question: When is it okay to point out mistakes? I have an orchestration book that looks as though it was self-published and got very little or no proofreading or editing before it was printed. Typos all over the place. Plus, many sentences are garbled and don't make a lot of sense. I get a sense of what the author is trying to say, but it isn't very clear. As a user of the book, do you think it would be rude for me to write to them and point out the errors? It's a useful book otherwise, for someone in my field (film composing).
Get a group of authors together, pour a few beers into 'em and they'll tell you what they think about "correction letters." They hate 'em. A lot.
You have the best of intentions: helping someone fix errors.
The key piece of the civility connection though, is this: by and large, once it's in print, it can't be fixed. If there is a second edition, MAYBE it can be, but second printings ARE NOT second editions. Second printings reprint the first edition, lock stock and typo. And authors have almost zero control over this. I've been very fortunate to have the one publisher who found an egregious mistake agree to fix it with zero guff. But then, that's why I like working with her..she really cares about the books on her list, and she's the decision maker about that stuff. Frequently editors at larger houses are not, and they're not real happy about mistakes either and there is nothing they can do about it.
The other connection you're missing is that most likely the author has already seen the mistakes and is a quivering heap of crushed word hoard on his/her agent's carpet, clutching the snake and whimpering "what did they do to my book?" Pointing out the errors when you see them is usually about two months after the author has and all it does is send them back to that whimpering thing. Trust me, that's not a pretty sight.
If you really feel the need to discuss rampant errors in a book, write a blog post about it. Link to the book on Amazon if you'd like. Trumpet your concerns about the failure of modern writers/copy editors/editors/agents to the world. Just don't write to them, or me, about it. We've already seen it and wept bitter tears.
And if you don't think this happens on EVERY SINGLE BOOK, think again.