The major theme in this book is not that history books glorify America. It is that history is taught (by teachers and by textbooks) in ways that make history uninteresting to students. Loewen argues that this is a major reason why Americans have such a poor understanding of history.
You are right to point out that one problem is that the books glorify America. However, this is not the major problem that Loewen sees. He thinks that history books concentrate too much on facts and figures and obscure the important things about history. They do not, he says, teach us about the connections between the past and the present. They do not focus on causes and effects. They tell us too much of what happened, but not enough about why things happened.
Loewen's main theme in this book is that the textbooks used to teach American history are written in such a way as to make history boring to the average student.