Another thesis statement about Lowen's work could be about embracing an inquiry based position regarding historical truth. One of the powerful elements that arises from the book is the idea that individuals must be able to formulate critical thinking and inquiry based skills in understanding the idea of truth in history. This requires us to be able to compare biases, examine points of view, and be ready to question our own senses of self in light of historical reality. A thesis statement could be constructed along these lines: "Lowen's idea of questioning the traditionalist approach to history brings to light a new manner in teaching the discipline." Essentially, I think you need to determine what is it about Lowen's work that you want to examine. Invariably, assessing how history is taught and received will be a part of this process.
It is not quite clear to me whether you are asking for a thesis statement for an essay you would write about this book or if you are looking for what the book's thesis is.
The book's thesis is that American history is taught poorly. This is because the books are filled with facts rather than ideas and because the books are too, essentially, conservative.
If I were writing an essay about this book, my thesis statement would be something like this:
Although James Loewen is right that history is taught poorly, I do not believe that the solution is to replace what he sees as conservative excess with his own excessively liberal ideas.
The afterword pretty much lays out his entire thesis.