Helseth destroys the fifty-years-old paradigm about the Old Princetonians (esp. Archibald Alexander, A. A. Hodge, B. B. Warfield, and J. Gresham Machen) that postconservative evangelicals continue to use today.
I’ve heard John Woodbridge rave about Helseth’s research on the Princetonians many times. Woodbridge initially encouraged Helseth to write this book, and Woodbridge’s foreword sets the historical stage for how unusual and bold this book is.
You can view a PDF of Woodbridge’s foreword (along with endorsements by scholars like John Frame, Roger Nicole, and Steve Nichols) here.