>>116087
The especially fun part of doing that kind of research is that not only does it help you spot bullshit, but it really helps you appreciate when the author gets something right, especially when it's subtle.
Given how much of isekai is at least loosely riffing off Medieval Europe, my go-to starting point for anything medieval is C.S. Lewis' The Discarded Image: An Introduction to Medieval and Renaissance Literature. It attempts to lay out the overall medieval cosmology as it was generally perceived by artists and average people, and introduce the reader to some of the less obvious late Greco-Roman and early Christian influences on it. Given how interconnected the medieval model of the universe was, it inevitably covers fields such as medicine, astrology, alchemy, the fey, magic's relation to language, and so on along the way. Not only that, but it does an excellent job of conveying just how differently many of these things were viewed back then from how they are now, and drawing out subtleties which are easily overlooked by the contemporary reader.
If you want to spot places where a medieval fantasy writer (eastern or western) truly gets the medieval mindset and how they viewed the world, I can't recommend this book enough. It won't help with the material past the way >>115954's familiar