Keep in mind that any source can be biased or otherwise flawed. Any book, magazine, newspaper, or website may be produced by a person or organization with a distinct bent on things. If you are writing an essay on the . Civil War and all of your sources are staunch Confederates, your essay will have a significant bias. If your essay focuses on parapsychology and all of your sources are tabloid newspapers, you're probably missing some important scientific writings both for and against the topic. It's okay to use a biased source, so long as you recognize that it is biased and counteract it with an opposing or neutral source. If you have good reason to suspect bias, be sure to present that as part of your analysis of what the source has to say.