I liked Bagley's work as the product of years of effort and the assembly of some new material. However, the book is very weak in the assessment of official documents, probably its greatest defect.
When Blood of the Prophets concludes that the LDS Church reached a "deal" in September 1876 with the Justice Department which would require the government to cease any further investigation of the massacre in exchange for the scapegoating of John D. Lee, Bagley misses two important things. First, he ignores federal case law which would have made any such deal a nullity and unenforceable. A federal prosecutor cannot offer a deal like the one Bagley describes without the approval of a judge or a president.
Second, he ignores official correspondence from 1876 to 1884. In that correspondence, government lawyers express the feeling that it would be wise not to make their investigation public, as it would alert possible suspects. The investigation, in the end, proved ineffectual. Nonetheless, the government pursued it for years. A president, a secretary of war, three attorneys general, several marshals, and a federal judge all weighed in on the prosecutorial effort from 1876 to 1884. A presidential pardon was secretly offered Lee to turn against Brigham Young in 1877 -- months after the date Bagley tells us a deal was made to ignore the prosecution of Brigham Young and others. A reward was offered by the Department of Justice in 1884 for the apprehension of massacre perpetrators who were fugitives.
Bagley's theory of a "deal," however, is the central theme of the book.
There really is, not yet, any definitive treatise on the massacre which adequately handles the official documents.