This book does not really prescribe a role for historians. However, the general message of the book can give us hints as to what role a historian could have in the United States today.
The general message of this book is that history is not an exact and objectively knowable thing. Arnold notes that historians have to pick which stories they are going to tell about history. They cannot do this on a completely objective basis. More importantly, they cannot know for sure that their stories are completely accurate. This is because they are not able to run experiments to determine whether those stories are right.
What this tells us about the present is that historians could help us to be more humble about the things that we think we know. I would argue that many people in the United States today are excessively “sure” about the things that they “know.” Historians ought to be able to help people understand that this feeling of certainty is misplaced. We simply cannot know the past (or many things about the present) accurately enough to feel that we should act zealously upon our understandings.
Thus, I would argue that the role of historians in today’s United States is mainly to impart to us the idea that our knowledge is limited and uncertain and that we should not be too arrogant about how we use that knowledge.