Sumerian is a language with no known relatives, and it was recovered because the Babylonians and Assyrians who inherited the writing system treated it as a classical tongue and produced bilingual material for their own students. Modern understanding of the oldest pantheon in the atlas rests, in large part, on ancient scribal homework.
The literary corpus comes overwhelmingly from one place and one moment: the scribal schools of Nippur in the early second millennium, where students copied a curriculum of hymns, disputations, laments and narratives about the gods. This means the Sumerian literature that survives is what was being taught several centuries after the culture that produced it was at its height, selected by teachers for what a scribe ought to know.
Alongside it stands administrative material in enormous volume, tens of thousands of tablets recording offerings, rations, personnel and temple estates. These are dry and they are decisive, because they show which gods actually received support, at what scale, and where, independent of what any hymn claims.
God lists, which pair deities with explanations and equivalences, add a third layer: the pantheon as the scribes themselves systematised it.
Two limits are worth stating. Much of the corpus is still being edited, and readings change. And the ordinary religion of people outside the temple economy is largely invisible, because it was not the kind of thing anyone wrote on a tablet and filed.