During the Middle Ages, most people were taught to read using the Psalter as their textbook. When vernacular books became common, vernacular translations of the Psalms were used.
However, it turns out that a lot of this was thanks to good old St. Senator Cassiodorus. His Commentary on the Psalms (more complete even than St. Augustine’s) included an insistence that a good chunk of the liberal arts were “hidden” in the Psalms. So he spends a lot of time pointing out all the important rhetorical and poetic devices used in the Psalter: metaphor, similes, structures, approaches to making your case, etc.
Mind you, he also spends tons of time on the religious content of the Psalms (of course), but his insistence on passing on classical info that might otherwise have been forgotten or misunderstood also had a big effect on the Middle Ages. You would be surprised how often, these days, one encounters Christians who don’t understand the rhetorical devices used in the Bible, and thus come up with bizarre interpretations, or decide that some passage is obviously “not meant” for them to read. In secular circles, the analogy sections actually have been removed from standardized tests, because kids no longer have either the vocabulary or the understanding of analogous reasoning, and often the question-writers had shared their lack. So this is still a very relevant concern.
Cassiodorus’ commentary is available in three volumes as Explanation of the Psalms, translated by P.G. Walsh, in the “Early Christian Writers” series by Paulist Press. But they’re only printed in hardback, so buying all three would set you back about a hundred bucks (or less, depending on where you shop online). If you live near a Catholic university or other well-stocked library, borrowing is good.
Of course, the medieval thing to do is to grab any bits you like and copy them over for use in your own material. 🙂