This is from the concluding paragraph of the review of Robert L. Millet. A previous professor at BYU reviewing the book.
Under the Banner of Heaven is not only a slap in the face of modern Latter-day Saints but also a misrepresentation of religion in general. It is an insult to those “unreasonable” beings out there who rely upon the “murky sectors of the heart and head that prompt most of us to believe in God — and compel an impassioned few, predictably, to carry that irrational belief to its logical end” (xxi). We should not be surprised that an author who begins his work with the statement that “faith is the very antithesis of reason” (xxiii) should thereafter proceed to grossly stereotype and thereby marginalize Mormonism.
I would not trust the book as unbiased. Here is the gospel topic essay on the Mountain Meadows Massacre.