Scripture and Creation are the pre-eminent revelations of God, or rather the pre-eminent revelations that aren’t confined to a historical moment, i.e. the Incarnation and the various theophanies that have occurred throughout time.
Creation can thus be understood as a sort of book in its own right. As such, there is a surface level of appearances, analogous to words on the page, and, beneath, an endless depth of hidden meaning which points back to the creator (incidentally, this is why the death of the author is an illegitimate mode of reading)
Our books, therefore, properly understood as sub-creations in the Tolkienian sense, ought to aspire towards a reflection of the world in both form and content. I contend that medieval thinkers explicitly understood this and consciously sought to affect it in their writings.
(Evidence of this is perhaps seen in their understanding of the Categories as a metaphysical work)
We might view the structure of things as an endless series of mirrors, all reflecting the primordial light of God’s self-giving love. The text mirrors the world, and expressly seeks to capture it as in a mirror, and thus the theology of creation and the conception of writing must ultimately self-reflexively mirror each other.
Thus, Muir’s comparison to a palimpsest is especially appropriate.