Having non-biblical readings has become such a canon among mainline Unitarian Universalists that Unitarian Universalist Christians face a crisis on the subject of readings. Is it proper to have non-biblical readings in worship?
The question of authority isn't clear-cut. My home library has several works of daily readings: selected sections meant to be read regularly to enrich one's faith, and not just in private reflection. Robert Atwell, the compiler of one such work (Celebrating the Seasons) notes in the introduction (page iii.) that
In monastic custom… the Scriptural reading at Vigils was supplemented by a non-Biblical lection. In the words of St. Benedict's Rule: 'In addition to the inspired words of the Old and New Testaments, the works read at Vigils should include explanations of Scripture by reputable and orthodox writers.' The reading of commentaries (presumably on what had just been read) enabled the monk not only to engage with Scripture more intelligently, but also to place his personal meditation within the context of those of other Christians from different ages and traditions.
We're not monks praying Vigils, but in our liberal-Reformed tradition we insist on the considered and thoughtful expounding on the lessons in the sermon. The lesson does not disclose itself, and we rely on the preacher to unfold its meaning.
In this sense, the non-biblical reading acts -- or could act -- as a replacement for the sermon, not the revealed word. But current Unitarian Universalist practice is far removed from this. When -- about a century ago -- Unitarian and (to a lesser degree) Universalist ministers cast abroad for non-biblical preaching texts, they drew from weighty stuff: often the classics, or a work of philosophy, or -- as a standby -- a bit of Shakespeare.
But today, it's not uncommon for a liturgical element from the back of the gray hymnal, or a segment from a ministerial contemporary to be pressed into the role of scripture. It an odd thought that a minister might visit a church and hear her or his words -- not unjustly quoted within the sermon -- elevated to the role scripture once held. It's hard to shake off our flippant and shallow reputation if that's the norm.
So, there may be a place for non-biblical readings in Christian worship, but to help us hear and understand the word of God: not to become it.