Forest Pyle suggests that Shelley's A Defence of Poetry--written soon after Epipsychidion--is an important document because Shelley treats poetic language "as a material force of worldly legislation in a history understood as the strife-ridden and discontinuous conformation of cultural and social forces," and as "the articulation of language and history, an articulation produced through the transformative agency of imagination" (95).
In her book Romanticism, Pragmatism, and Deconstruction, Kathleen M. Wheeler points to Shelley as being the most revolutionary poet of the romantic period. While Coleridge laid the groundwork with his primary-secondary imagination distinction--with the secondary imagination being involved in the reorganizing of materials for the purpose of art creation. Shelley, according to Wheeler, went "beyond Coleridge's account" due to his "focus upon metaphor as the vehicle for knowledge, whether in science, philosophy, or art," thus anticipating "modern philosophy of science" (7-8). Moreover, Shelley "took Coleridge's primary/secondary imagination distinction and reinterpreted it accurately [...] as the crucial analogy existing between basic perception and artistic (and poetic) perception" (8).
I think we can all see where Wheeler is going with these remarks: that Shelley may have been anticipating such figures as Nietzsche, who upset epistemological assumptions about truth. After reading the first sentences of "On Truth and Lie in an Extra-Moral Sense," we can see that he is advancing the idea that there are both truths and lies wrapped up in language, that language is by no means a method of communicating divine truth.
Pyle, Forest. The ideology of Imagination : Subject and Society in the Discourse of Romanticism. Stanford: Stanford UP, 1995.
Wheeler, Kathleen M. Romanticism, Pragmatism, and Deconstruction. Oxford: Blackwell, 1993.