Warning - I'm by no means equipped to write very much about this subject; this post will be for the most part merely a reference for a few notes about it.
David Knowles devotes a chapter to St. Augustine in his book The Evolution of Medieval Thought (as an aside, there are some preposterous prices for this book on Amazon; I paid seventy-five cents for a copy in average condition, but some folks are apparently asking as much as $240.00 for it! I'm obviously in the wrong business - or maybe those sellers are :-) It's a great book - but there is no way I'd pay $240 for it). He doesn't do this because he considers Augustine a medieval author; rather, he does so because of Augustine's tremendous influence on medieval thought.
Knowles explains that St. Augustine's epistemology is a difficult subject, owing partly to the fact that he knew very little of Plato's writing and none of Aristotle's (p. 40). He seems to have been influenced more by Plotinus: "There is a practical rule which rarely fails the commentator on Augustine's philosophy: it is that when a source for his thought is wanting the Enneads of Plotinus should be searched" (p. 43).
Knowles writes that (although Augustine is not terribly consistent about it) for the most part the saint viewed cognition as driven primarily by divine illumination of the intellect (p. 40f). For Augustine "all perception and knowledge arises within the soul; the soul is not acted upon directly by the external world, nor does it 'abstract' anything from that world in the process of cognition" (ibid).
Knowles remarks that there is sharp disagreement among scholars about St. Augustine's epistemology, and since I can barely wrap my head around it, I'm going to leave it at that for now.