I've noticed that reading old texts with alien mindsets is an instant idea generator for fantasy settings. (Seriously, I don't get I've never heard anyone suggest the notion of "fantasy writers should read old texts" before. They're just filled with peculiar ideas about the world that one can import directly to a fantasy setting.) Would you happen to have any recommendations on texts that would be particularly suitable for this?

That is a great question. The first thing to come to mind actually isn't all that old, but from the Early Modern Period. To a large extent, the Renaissance was much more mystical than the Late Middle Ages (seriously, compare Galileo's Platonism to Swineshead's Scholasticism). The paradigm I'm referring to is usually referred to as the "natural magic tradition" and is exemplified by thinkers like Paracelsus (The Hermetic and Alchemical Writings of Paracelsus) and Heinrich Cornelius Agrippa von Nettesheim (Three Books of Occult Philosophy).

The SEP entry for Agrippa sounds downright HPMOR-esque:

De occulta philosophia in its early form showed Agrippa's determination to transform magic into a useful science that would draw together all branches of magical learning, set those materials into a single philosophical framework, purge magic of the evil and demonic practices that had caused it to be regarded as a wicked science, and turn it into knowledge that would be beneficial to humanity. His goal was a total regeneration of magic, transforming it into a science that would enable the magus, or learned practitioner of magic, to perform marvelous works that would contribute to the welfare of humanity (Kavey 2010)....

Such learning is esoteric. Because of the power it confers, it would be potentially dangerous to religion, society, and individuals if it fell into the hands of the crude and ignorant masses. It must be communicated only to individuals whom the magician (the magus) knew to be worthy, both intellectually and morally, people who would use this power for the benefit of humanity (OP 3:2).