These contradictions and more provide the narrative tension for ''Mormon America,'' a long overdue primer on one of the fastest-growing religions in the world -- officially, the Church of Jesus Christ of Latter-day Saints.  This home-grown faith, founded by an itinerant treasure digger from upstate New York in 1830, has grown to a world membership of more than 10 million, including five current United States senators.  Richard N. Ostling, a religion writer for The Associated Press, and his wife, Joan K. Ostling, a freelance writer, note that if it continues to add followers at the present rate, the Mormon Church could become the most important world religion to emerge since the rise of Islam 14 centuries ago. 

At the start of the 21st century, Mormons project a toothy wholesomeness.  But the Ostlings remind us that this supposed white-bread religion was founded by militia-backed authoritarians who nearly went to war with the United States and prompted the first Supreme Court rulings to restrict religious liberty.  Joseph Smith, the Mormon prophet and founder of the church, was killed by a mob in 1844, but only after he had declared martial law in Nauvoo, Ill., and ordered his followers to smash the printing press of a publication that had reported on polygamy among church leaders.  Of course, many religions have morphed their questionable pasts into iconic nostalgia.  The Church of England, after all, was founded by a serial killer, King Henry VIII, who was unhappy over institutional moralizing about the way he disposed of his wives.  But the Mormons are burdened by being one of few major religions subject to recent fact-checking.  Many of the foundations of the church -- among them, the beliefs that American Indians were Jews who sailed across the Atlantic, and that the biblical Garden of Eden was really a verdant patch of Missouri, in what is now a parking lot near Harry Truman's home -- are embedded as divinely inspired, historical facts in the golden-plated Book of Mormon brought forth by Smith.  Few non-Mormon scholars have ever considered these claims to be anything more than archaeological fairy tales.  DNA analysis, for example, has failed to establish a viable connection between the Hebrews of old Israel and the native peoples of the Americas.  ''Book of Mormon apologists have a much tougher job than apologists for the Bible,'' the Ostlings write.  ''Not a single person, place or event unique to Joseph Smith's 'gold Bible' has ever been proven to exist.'' 

Still, for all the harsh lighting of historical fallacies, ''Mormon America'' should not be mistaken for a polemic.  Most books on the Saints, as they call themselves, tend to be anti-Mormon screeds or soft-focus proselytizing.  This book is eminently fair, well researched and exhaustive.  There are no major revelations that are likely to alter opinion one way or the other, but the authors are diligent referees of fights past and present. 

In the opening chapters, the Ostlings dramatically recreate Smith's rise and the events leading up to his death at 38 in the doomed Mormon city of Nauvoo.  Though the story has often been told, it is still full of tension, hubris and fatal misunderstandings.  Whether Smith was a latter-day prophet or simply a charismatic fraud, the Ostlings never conclude.  He can be viewed as the Henry Ford of religion, with a uniquely American knack for enterprise and innovation, or a Jim Jones figure from the dark edge of the frontier, ready to take his followers off a cliff with him. 

In the second half of their book, the Ostlings try to sort through the many nuances of church doctrine.  Heaven has three tiers, a spiritual class system that appears to be much easier to get into than the hereafter of other religions.  Hell is reserved only for the most evil.  Blacks were long considered lesser human beings until a public outcry prompted a 1978 reform that allowed them to become priests. 

Through it all, the Mormons have been ever practical, adaptable and optimistic.  The church president is also the Prophet, Seer and Revelator -- a direct spokesman for God.  He can shed sacred Mormon beliefs when they no longer suit the times, and doing so has never seemed to bother most followers.  Ultimately, as with any religion, belief comes down to faith. 

The weakness of ''Mormon America'' is that it does not go into much depth on the temporal world created by the Saints.  The physical and political realm, the empire that sprouted from the original plan for a Zion in Utah and is enlarged by the annual 10 percent tithings of all members in good standing, is subject to mere listings of holdings and new buildings.  It is on the spiritual front that the book most comes to life.  Quoting from dozens of anguished Mormon scholars who have run up against the iron fist of church authorities, the Ostlings make the case that the Saints should not be afraid of their own past.  People who buck the party line are spied on, denounced and coldly excommunicated.  ''No other sizable religion in America monitors its own followers in this way,'' the Ostlings write.  Other religions also rein in their freethinking renegades, but the Mormon Church ''is unusual in penalizing members for merely criticizing officialdom or for publishing truthful -- if uncomfortable -- information,'' the authors say.  This raises the question of whether Joseph Smith himself, were he alive now, would last long in the present church.  Like many religious visionaries, he was a heretic in his day. 

Timothy Egan, who writes on national issues for The New York Times, is the author of ''Lasso the Wind: Away to the New West.''