Michael Kinsley opens an amusing Washington Post column about newspaper corrections by noting that "'fact checking' is a tradition of some publications, mainly magazines, in which one set of employees, called fact checkers, is called upon to reconfirm every fact in an article by another set of employees, called writers, generally by finding these facts in newspapers, which don't have fact checkers." He might have added that fact checkers also confirm facts by looking them up in books, which generally do not have fact checkers either.
When I was a "reporter-researcher" at Fortune during college, the Time Inc. policy was that one verification by book was worth two verifications by newspaper. If I'm remembering the color scheme correctly, the former was a "red check," which was good enough on its own, whereas the latter was a "black check," acceptable only in pairs. I may have the colors reversed, but the point is that we all assumed books were more reliable than newspapers (or other periodicals). That was a mistake, I think. While some books categorized as nonfiction, such as reference works and peer-reviewed releases from academic presses, go through some sort of fact checking, the vast majority do not. (They are generally reviewed by lawyers with an eye toward possible libel issues, but you can get lots of things wrong without risking a lawsuit.) In fact, judging from the finished products, I'd say most books are barely edited, let alone checked for accuracy. By contrast, newspaper stories typically are reviewed by two or three editors before they see print. It's true that books take longer to produce, which gives a conscientious author more time to catch mistakes. Then again, they are a lot longer than newspaper stories, so there is more room for error.