This is an excellent account of one of the most significant Christian leaders of the 20th century. It is not a biography, because Hart restricts the account to the events and issues surrounding Machen's involvement in the Presbyterian controversy over confessionalism and modernism that led up to the reorganization of Princeton Seminary and the founding of Westminster Seminary in 1929, and the subsequent founding of the Orthodox Presbyterian Church in 1937. However, Hart does give a lot of attention to the cultural and ideological background that informed Machen's writings and actions during that tumultuous period, and so I did finish the book with a much deeper picture of who Machen was.
Hart examines Machen's family background and especially the context of Southern Presbyterianism on his mother's side of the family. He examines Machen's paradoxical position in the debate surrounding Biblical criticism, which was that while he was squarely in the tradition of confessional orthodox Presbyterianism as opposed to the higher critical schools of theology, he was still eager to exploit all the methods and results that the latest scholarship could provide. In his academic position he wrote scholarly books that defended such doctrines as the virgin birth of Christ, but were nonetheless highly regarded and favorably reviewed by his modernist academic peers because of the high level of scholarship that he was able to bring to bear. For understanding this aspect of his thought, Hart discusses his educational background, and especially his experience studying in Germany and his personal crisis leading up to his accepting a position at Princeton Seminary. Hart also spends a considerable part of the book examining Machen's relationship to the rest of the fundamentalists, especially on confessionalism, evolution, and the role of the Church in society, where Machen diverged from most of his fundamentalist contemporaries.
The most interesting part of the book for me was the discussion of Machen's political views and how he saw parallels between the events surrounding the crisis in the Presbyterian church and the developments in American political life. Machen was a full-fledged libertarian politically: he opposed Prohibition (a deeply unpopular view among both fundamentalists and theological liberals at the time); he opposed the creation of a Federal Department of Education, and he also opposed the rise of the welfare state during the New Deal and the Great Depression, which he (rightly) saw as coercive wealth redistribution by the state and a denial of private property. In all these things, he opposed and feared the expansion of state power as the greatest political threat the church faced. He was also aware that a parallel centralization of power and bureaucratization of the church hierarchy was a major part of the driving force behind his opponents in the mainline Presbyterian church. Many are aware of the theological issues that Machen stood for. I don't believe that nearly so many are aware of his social and political views, which were intricately connected to his theological views and especially his view of the role of the church. It is a shame that the Reformed Church seems to have ignored that aspect of Machen's thought.
I highly recommend reading this book to get a better understanding of the issues and ideas that were influential for Machen. It is scholarly but accessible, and it also has an extensive bibliography at the end. I would recommend reading Longfield's book "The Presbyterian Controversy" first, because it gives a broad overview of the period for context.