Books - It can take up to two years for a book to be published, so the information in a 1999 book may already be out of date. Information that does not conform to these criteria is not necessarily flawed or unreliable, but you should use it with caution. Remember that the web in particular is open to anyone who can write anything they want. Coverage When considering whether an information source is going to be useful you need to look at the range it covers. You also need to consider whether it consists of primary or secondary source material.
Primary material contians new information or a new interpretation of previously known information.
Secondary material is interpretation and comment on primary material by others. You may need to cover a variety of different viewpoints. If your essay is on a broad topic don't try to absorb every detail you can find. Start with an article from an encyclopedia or find a book that gives a general overview of your topic. When you need detailed information an academic article is more likely to help you than a general overview.