Here are some online pamphlets related to EP posted at the RP Archive. Many helpful articles!
Christian Worship by Gene Spear
A Condensed Argument for the exclusive use of the Inspired Psalms by R. J. Dodd
The Excellence of the Psalms by D. B. Willson
Instrumental Music a Corruption of New Testament Worship by R. J. George
Instrumental Music in the Worship of God
Is Christ in the Psalms? By William J. Coleman
Musical Instruments in Divine Worship by W. J. McKnight
Psalm Singing Revisited by Bruce C. Stewart
Psalmody by Roy C. Fullerton
Psalmody by R. J. George
The Psalms, a means to a Spirit filled life
The Psalms, God’s Authorized Manual of Praise by A. J. McFarland
Psalms, Hymns, and Spiritual Songs
The Psalms, the Heart of the Bible
The Singing of Psalms in the Worship of God
Why no Instruments? By Robert B. McCracken
Why Psalms? By G. I. Williamson
Exclusive Psalmody 