When we talk about "credible" sources, we mean those which are accurate and trustworthy, coming from a reputable authority. If you work within the library databases, you don't have to worry about this as much, because information sources have to be selected for inclusion in the database collections. Google, on the other hand, isn't picky, and it doesn't care about accuracy -- which means you have to work a little harder to get good information.
Wait, are these credible or or are they scholarly?
Books are just a medium rather than a class of information. There are scholarly books, there are credible non-scholarly ("popular") works, there are reference works (e.g. encyclopedias -- which are not usually appropriate for your final works cited), and there's junk books. Nonfiction is not the determiner of scholarly.
Scholarly books, like scholarly articles, will be authored by subject-matter expert (PhD not hobbyist), provide copious citations, and will be published by a university press (e.g. University of Texas Press) or a professional society (e.g. the American Psychological Association).