The John Newton Project published a book by Newton on pastoral ministry that was previously unpublished: Ministry on My Mind: John Newton on entering pastoral ministry (transcribed by Marylynn Rouse, Stratford-upon-Avon: The John Newton Project, 2008). This book contains Newton’s thoughts on entering the pastoral ministry, and was written at Liverpool in 1758. You can see the book on the main page, but you need to go here to buy it. They provide several blurbs for the book, one of which is J. I. Packer:
“It is hard to believe that any Christians, wondering if God was calling them to ordained service, ever meditated on relevant Scriptures so perceptively, and recorded their discernments so luminously …a very precious part of the legacy of this great man of God.”
I also found a review of the book by Michael A. G. Haykin in pdf form. Haykin begins his review by explaining:
In the history of the Church, there are a number of classical studies on the nature of pastoral ministry that anyone aspiring to the office of pastor has to read: Gregory of Nazianzus’ Oration 2 (362 AD), for example, or The Reformed Pastor by Richard Baxter. John Newton’s Ministry on my Mind—not Newton’s title, but that chosen by Marylynn Rouse—now available for the first time since he wrote it in 1758, makes for a good addition to these classical studies of what it means to be a pastor.