The Mechanics of Ancient Egyptian Magical Practice by Robert Kriech Ritner is a curious work that deals with the religious nature of Heka – speech as the personification of magic. The fragments of Heraclitus by Dennis Sweet are also well worth pondering, dealing with the divine nature of Logos.
Mysticism isn't necessarily irrational, though. Plato's dailogues should be considered a conglomation of the Mediterrenean and Near Eastern mystery traditions, systematized, refined and expounded in a rational whole suitable for the Greek intellectual milleu.
Plato deals with language in several places, most notably the Cratylus where he discusses the etymology of the Homeric gods and the nature of rhetoric in the Phaedrus, which he calls psychagogia, or soul leading, a title bore by the Greek Goes, or "Shamans". I would like to recommend the Enneads of Plotinus in MacKenna's translation, but you'll need a firm grasp on Plato's corpus to really appreciate him.
As to the relation between poetry and the divine, Plato introduced the concepts of the divine frenzies in his Timaeus, of which there are four – the poetic frenzy, inspired by the Muses, the prophetic frenzy, inspired by Apollo, the telestic frenzy, inspired by Dionysos, and the erotic frenzy, inspired by Eros.
The Defense of Poetry by Percy Bysshe Shelley and On Poesy or Art by Samuel Taylor Coleridge are well known for their elevation of the poetic frenzy. In general, the Romantics are much more accesible than the Renaissance Platonists.
In regards to the "direct confrontation with God or the Universe without any sort of mediator, without language or poetry", this sentiment is best expressed in On Learned Ignorance by Nicholas of Cusa, whose full corpus is availiable at the site of Jasper Hopkins.
I hope this has been helpful.