With reference to A People's History of the United States, what does "anyone reading history should understand from the start that there is no such thing as impartial history" mean?
As far as I can tell, the sentence you quote does not actually appear in this book. However, it is still relevant to Zinn’s book and to anyone reading it. This is because Zinn can easily be accused of writing history that is not impartial.
What this quote means is that any historical account that is at all meaningful will have to show the biases of the person who writes it. It is not possible to simply state objective facts in a historical account that matters at all. For example, you can say “Barack Obama won the 2012 US presidential election,“ and that will be an impartial and objective statement. However, it really doesn’t tell us much. Once you start asking why Obama won, or what his victory means, you start getting into opinions and you will have to select among these opinions when you write your history. Once you start picking some things to include and others to leave out, you are no longer impartial. You will, at the very least, be allowing your biases to influence you when you decide which things are important to your story.
This is relevant to Zinn’s book because Zinn can be accused of partiality. The facts that he chooses are meant to support his belief that US history has been full of injustices caused by the desire of the ruling elites to retain power. However, the quote reminds us that any “regular” history of the US has also been written with a certain biased point of view and so we should not discount what Zinn says.