Y ou must always question your sources. You must always question an author's motives. Never take the information at face value just because it is in print.
Writers publish their ideas based on their own interpretation of primary sources.
In any event, certain ideas and events would be highlighted while others wouldn't. Therefore, when writing about a topic, realize secondary sources have already been filtered by someone else's point of view. However, even a primary source must be questioned in terms of point of view. For example, the diary of a WW II veteran may represent an individual who thrived on the glory of battle and nothing else. In any event, this is one point of view about the war and may not represent the majority of those in the ranks.