It was pretty obvious that some authors didn't really know what they were talking about. They knew enough to write something, but there were always a lot of mistakes.
This is why I'd be very hesitant to self-publish a technical book, even though for fiction I think self-publishing is the right decision 99% of the time. We all make errors, even those of us who do know what we're talking about, when you get to the scale of 100+ kilowords. For a novel, a decent copyeditor can fix up the production values well enough; for a technical work, you would hope the publisher assigned some people to check the work.