Finding reference materials to use isn't the only important part of any research. You also need to determine the reliability of that material.

One of the most basic things to do is to check the copyright date of a book you intend to use. While this information won't be as critical in a book about the Civil War, it can make a difference if you need to write about something more current. A book about breast cancer treatments published in 1990 may still include what will be a great deal of useful information, but you can't rely on it for details on treatments in use today.