Different papers and assignments call for different resources; for some projects, blogs and more casual offerings suffice, but other (if not most) tasks require quality and reliable scholarly sources. This page offers a few helpful sites and pieces designed to aid in identifying an appropriate academic resource/source.
An important step in evaluating a resource is determining if it is considered scholarly. This distinction will need to be made for books, as well as articles.
If you find that you are still having difficulty, ask your professor or consult with a librarian.