The Hymn to Demeter is the fullest surviving account of the rape of Persephone, of Demeter's search, of the bargain over the pomegranate seeds that divides the year, and of the goddess's stay at Eleusis. It ends by describing the founding of rites there and by warning that they may not be spoken of. That combination, a narrative charter for the Eleusinian Mysteries written by someone who will not disclose them, makes the hymn the single most important literary source for the most famous mystery cult of the ancient world, and also a demonstration of how little literature can tell us about it.
The Hymn to Apollo, which the ancients may have known as two poems, tells of Leto's wandering, the god's birth on Delos, and his founding of the oracle at Delphi after killing the serpent there. Its poet calls himself a blind man living on Chios, one of very few first-person self-descriptions in archaic Greek poetry. The Hymn to Hermes is comic: the god steals Apollo's cattle on the day he is born, invents the lyre from a tortoise shell, and talks his way out of trouble. The Hymn to Aphrodite tells of her seduction of Anchises and of her humiliation at being made subject to the desire she imposes on others.
Because Greek religion had no scripture, these poems are not authoritative in the way a canonical text is. They are the best surviving specimens of a very large body of cult poetry, and they show what a Greek audience expected to hear about a god at a festival. That is a different thing from doctrine, and the difference matters when they are read beside the scriptures of traditions that have one.