The other day, I was telling a group that it is so important to analyze evidence (i.e., documents, newspaper clipping, books) that you gather for your family history research. One good example is 19th century books. During this time period, it was not uncommon to plagiarize large sections of other people's books.
Also, these works were not cited so they could basically claim anything in their non-fiction book. So in using a non-fiction book from the 19th century, you may want to find additional sources to back up any claims in the book. A good example of books that may be less than truthful is county history books or sometimes called "mug books".
On the website for the Arizona State Library, Archives and Public Records they have a form entitled How to Analyze a Document. It's a simple 1 page checklist of sorts that helps you decide the intent of the document. I like that 1 of the questions is "what biases do you find in the document?" Remember, everyone has an agenda.
To open the form you will need Adobe Acrobat, which is a free download. The form can be found at http://www.lib.az.us/archives/images/pdf/document_analysis_sheet.pdf.
They also have a form for analyzing a photo at http://www.lib.az.us/archives/images/pdf/photo_analysis.pdf.