When I find typos I tend to suspect the content is skewed also. Credibility is all a book has when you come down to it. Without credibility, no matter how important the message, readers won't believe it. When a fiction writer puts a wristwatch on George Washington or a business writer can't use simple English, the reader stumbles. Getting him back on track is nearly impossible.
Palm Island has a policy of no changes once the disk has been sent to the printer.
That's a tough policy, but wise. There has to be a point when the publisher is willing to let go, because there is no such thing as a perfect book. I was a technical writer and editor for ten years. For seven of those I worked on one four hundred-page manual published annually. I knew it inside out, dreamt about it, bored my friends with it.