This is an abridgment of a much larger work by Frazer that compiles, categorizes and interprets the belief systems of very old cultures. It's easy to get overwhelmed by the extensive listing of examples that Frazer provides unless these are viewed as attempts by these cultures to understand and control nature through magical practices. These practices for Frazer appear to manifest deeper structures surrounding human need and fear. In short, Frazer writes, they reflect "the essential similiarities of man's chief wants everywhere and at all times."
On this issue of causality, Frazer believes there's a progression from magic to religion to science. As the editor and abridger of Frazer, Gaster is critical of that point of view, stating that it "has now been shown to be a mere product of late nineteenth-century evolutionism, without adequate basis." But how Frazer's perspective is without merit is not clear. In his commentary, Gaster's general criticisms seem obtuse or nitpicky and, at times, he seems like he is trying to preserve a special place for religion, as distinct from primitive magic. Given this, it's hard not to wonder what of Judaism and Christianity Gaster left out of this abridgment. For Frazer's part, science is much in use these days relative to earlier times and that is likely his point, even though a good part of humankind still believes in forces from an unseen world that are propitiated by prayer and such. Gaster is also critical of Frazer's sources of information, suggesting that the Golden Bough rests on a somewhat flimsy ground, even though Frazer extensively documents his studies, drawing from well-known and well-cited sources (e.g., Pliny, Boas, Plutarch, Herodotus, Schoolcraft, Hume, Ovid, Livy, Plato). And, in contrast to Gaster's various authoritative assertions about Frazer's opinions, Frazer offers his specific interpretations with some humility, suggesting an openess to alternative explanations. "All our theories" of primitive man, Frazer states, "fall far short of certainty; the utmost we can aspire to in such matters is a reasonable degree of probability."
This (700 page, plus) abridgment is rich in detail on the practices of earlier humans and Frazer's interpretation of their meaning seems reasonable enough. In my view, they are more understandable and perhaps more probable than, say, Campbell's explanations.