The Regulative Principle of Worship, 2001, 12 pages
This is something of a briefer condensation of Williamson’s longer article below.
The Scriptural Regulative Principle of Worship, no date, 37 paragraphs, with two appendices 13 paragraphs long together
This is the best introduction to the Regulative Principle of Worship (R.P.W.). Williamson briefly exposits all the major Biblical passages that teach the R.P.W., including 10 from the O.T. and 7 from the N.T. The first appendix looks at how the historic, reformed church applied this Biblical principle of worship to particular acts of worship. The second appendix addresses the loose interpretation of the R.P.W. that is common today.
The Singing of Psalms in the Worship of God, no date, 58 paragraphs
A good introduction to the subject.