Who is the author? Is the author at a credible institution, like a University? If not, does the author have some other claim to expertise in the field? Who is the publisher? What other sorts of publications do they put out? Investigating those two questions can tell you quite a bit about the credibility of a source. Citing sources is standard and expected in academic publishing, though some very credible authors don't bother. If the book is more in the popular consumer realm, citations often get dropped.