This is a really excellent, scholarly analysis of the current best thinking on mystery cults in the Greco-Roman milieu. Bremmer is cautious in his speculations but lively in his judgments. He is not afraid to say "we don't know more than this" in giving reconstructions of the initiatory practices of a wide variety of cults, including Eleusinian, Samothracian, Orphic-Bacchic, Dionysiac, Mithraic, and Isian.
One of his great strengths is showing how local practice varied so widely, and teasing out the difference between cults of fixed place like Eleusis and the later itinerant practices of Orphic movements. In later chapters, Bremmer explores the evidence for cross-fertilization between early Christianity and the mystery cults active in the Empire. His conclusions are a sobering brake on many of the overheated syncretic claims of late-19th-century scholarship and its long hangover, while also teasing out some fertile connections.
I really savored his facility with textual analysis: from a single word, Bremmer can often trace a dense web of allusion that is largely occluded from our view here at this end of history. It's an exciting, playful, and deeply nerdy kind of scholarship, and it's a great joy to read even as an outsider. This text is generally pretty friendly to the dilettante, but note that Bremmer does not hesitate to use German terms and quotations without translation, along with the occasional passage in Latin. All Greek terms and most Latin terms are given English translations and exegeses. This is one of the better books on the mysteries I've found, even if some esotericists or modern pagans may find its conclusions a tad deflating. Highly recommended.