The Golden Ass is the only novel from classical antiquity to survive complete. It was written in the late second century by one Apuleius, and deals with the tragi-comic misadventures of a man who has been turned into a donkey as a result of a little too much interest in magic. It also includes the tale of Cupid and Psyche, as a tale-within-a-tale. It has a lot of magical events in, quite apart from the main character being turned into a donkey, and gets quite deeply involved with Roman mystery religions, particularly the cult of Isis, towards the end. It is, in other words, quite clearly a fantasy novel.
It’s also quite good. Standards for entertainment shift over time, and as this book is pushing 2,000 there are a few bits that don’t quite work by contemporary standards. Cupid and Psyche is about a fifth of the book, for example. Lucius, the main character, gets into some fairly entertaining bits of trouble along the way, and there are early versions of a number of classic comic scenes. But there are also tragedies. I think it is worth reading as a story, and very definitely worth reading if, like me, you are interested in historical beliefs in the supernatural.
Of course, the fact that it is a novel makes it a bit tricky as a source. It is generally agreed that some bits are supposed to reflect reality; some bits are independently attested. But when it comes to the stories of magic, it isn’t clear how much of that contemporary people actually believed. If you took, say, Anne Rice’s vampire novels as indicating what contemporary people believed, you’d get an inaccurate picture. At the time, it was probably obvious which bits were plausible and which just made up, but we don’t have that cultural background, so we are left wondering.
Fortunately, when writing for roleplaying games, you can generally gloss over such problems. If people told stories about that then, the elements are suitable for telling stories set then. And, of course, if you’re just reading the book for fun, you don’t need to worry about it at all.