Authentic Thaumaturgy by Isaac Bonewits is a system-agnostic RPG supplement with a long heritage; its original edition came out in the 1970s from Chaosium, whilst Steve Jackson Games put out an extensively revised version in 1998. Though it’s available in an e-book edition from Steve Jackson Games’ e23 storefront, I wouldn’t hold my breath for a reprint: not only did Bonewits die in 2010, but in 2018 Moira Greyland, daughter of Marion Zimmer Bradley, implicated Bonewits as having a peripheral role in the abuse that she suffered at Bradley’s hands.
Either edition purports to offer more or less the same thing: an analysis of tabletop RPG magic, a discussion of what real-world magical practitioners believe and do, and a proposal for a magic system based on real-world practices and the sort of effects people claim to get out of them. The whole thing is written from the perspective that the point of an RPG is to be a realistic simulation, and therefore Bonewits has little time for magic systems which don’t resemble real-life magical practices, even though such systems might be more suitable for, say, presenting a challenging and exciting game, or for providing an interesting feature or metaphor in a story. I, a highly trained scientist, can enjoy science fiction stories can be enjoyed even when the science involved has been discredited or was never that realistic to begin with: why can’t Bonewits do the same for fantasy stories whose treatment of magic doesn’t match his worldview?
And the thing is, that worldview is decidedly central to the book, and it’s rooted in the late-1960s/early 1970s neopagan scene that Bonewits emerged in. The thing is that Bonewits clearly has a highly developed personal ideology of what magic is and isn’t and how it works, and he seems to be personally offended by different ideas in the field.