Boorstin also warns us that much of what survives is not what people read. After all, a well-read book gets worn out. Also, he says to be wary of diaries, etc. These documents can be rather self-serving. Most famous people who write diaries are well-aware that they will be read one day.
Secondary sources have different problems.
The biggest problem is the bias of the author. For instance, Bill Shirer's, Rise and Fall of the Third Reich is a very good account of Nazi Germany. Another problem secondary sources run into is one of new evidence showing up later. A case in point is that many an art critic made a career out of showing that Michelangelo's greatness was in his dark, brooding portrayal on the Sistine Chapel ceiling. As it turns out, the darkness was a combination of dirt and glue.