Whether you are using library books, articles, or web sites, it is important to critically evaluate any information source before you use it in your research. The Everyday Writer (2001, p.111) suggests some guidelines that you should use to evaluate the sources you are collecting:

Relevance.
Is the source closely related to your research question?

Author’s and publisher’s credentials and stance.
Is the author an expert on the topic? What is the author’s stance on the issue(s) involved, and how does this influence the information in the source? Does the author support or challenge your own views? If you are evaluating a book published by a corporation, government agency, or interest group, what is the publisher’s position on the topic? If you are evaluating an article, what kind of periodical published it—popular? academic? alternative?

Date of publication.
Recent sources are often more useful than older ones, particularly in the sciences. However, in some fields, the most authoritative works may be the older ones.

Level of specialization.
General sources can be helpful as you begin your research, but you may then need the authority or currency of more specialized sources. On the other hand, extremely specialized works may be too hard to understand.

Audience.
Was the source written for the general public? specialists? advocates or opponents?

Cross-referencing.
Is the source cited in other works?

Length.
Is the source long enough to provide adequate detail?

Availability.
Do you have access to the source?