I can't praise Denise Levertov's New & Selected Essays (New Directions, 1992) highly enough. The collection contains 25 simply stunning essays of literary criticism and writings on the creative imagination and the poetic process. Really you have to read the whole essay to understand fully what she means, but Levertov says this about mystery and imagination in concluding A Poet's View:
This acknowledgement, and celebration, of mystery probably constitutes the most consistent theme of my poetry from its very beginnings. Because it is a matter of which I am conscious, it is possible, however imprecisely, to call it an intellectual position; but it is one which emphasizes the incapacity of reason alone (much though I delight in elegant logic) to comprehend experience, and considers Imagination the chief of human faculties. It must therefore be by the exercise of that faculty that one moves toward faith, and possibly by its failure that one rejects it as delusion. Poems present their testimony as circumstantial evidences, not as closing argument. Where Wallace Stevens says, 'God and the imagination are one', I would say that the imagination, which synergizes intellect, emotion and instinct, is the perceptive organ through which it is possible, though not inevitable, to experience God.
Levertov defines her poetic stance quite admirably and succinctly; I don't think I've ever read, except in Coleridge or Eliot, a better exposition of the subtle connections between logical intellect, mysterious imagination, faith and God.