Frame, John M. The Theological Correspondence of John Frame. Bellingham, WA: Lexham Press, 2017.
Booth, William. Purity of Heart: Letters to Salvation Army Officers. n.p., n.d.
Wesley, John, and Joseph Priestley. Original Letters, by the Rev. John Wesley, and His Friends. Birmingham: Thomas Pearson, 1791.
That gets everything that is clearly letters out of my monograph collection -- there are other letters but in resources that have mixed types.