Prolegomena to the Study of Greek Religion is not, despite the title, an introduction to Greek Religion. Beginners to the subject should look elsewhere, first, and return to this book later. Instead this is a study of the foundations (or alternatively depths) of Greek religion. It is not an easy read, nor intended to be, but it is an undeniable classic and amazing work of scholarship.
Most of us are aware of the classical stories of Greek religion, Zeus, the minotaur, wax wings etc. Instead in this book Jane Ellen Harrison focuses on the rituals, mysteries and other features of the older religion that gave birth to these myths. Rather than godly beings consuming ambrosia on top of a mountain these are beliefs of ghosts, demons and potentially vengeful spirits which must be placated, or disaster will occur. As Harrison writes "Great things in literature, Greek plays for example, I most enjoy when behind their bright splendours I see moving darker and older shapes." This work provides insights, showing us the darker, older shapes moving behind classical Greek religion. Once you understand it you will not see the old stories in the same light ever again.