Spurgeon and the Downgrade Controversy in John Macarthur's Ashamed of the Gospel took me by storm. It was also the book that "first sparked" Phil Johnson's interest in Charles Spurgeon. Perhaps he would not have built Spurgeon Archive if he has not read that book.
That appendix lead me to pick-up R.J. Sheehan's C.H. Spurgeon and the Modern Church and Iain Murray's The Forgotten Spurgeon.
Just last week, I was blessed to discover a paper written by Master Seminary's Dennis Swanson. He read this paper at the annual meeting of the Evangelical Theological Society in 2001 when the society's theme was Defining Evangelicalism’s Boundaries.
The paper is available in PDF format here.