As far as reading is concerned, what undoubtedly influenced me most, during the critical months of inner wrestling, to conclude that I could not “bend” the Sermon on the Mount and the whole concept of the Cross and suffering love to accommodate participation in war, was the serious reading of the Christian mystics. ~ A. J. Muste, ‘Sketches for an Autobiography,’ in The Essays of A. J. Muste, N. Hentoff, ed., Bobbs-Merrill, Indianapolis, 1967, p. 47.
Here Muste specifically mentions the work of Rufus M. Jones as being helpful. Many of his works are freely available on Archive.org.