The leading Sumerologist of his generation, at the University of Pennsylvania Museum for most of his career, and the man who made the Sumerian literary corpus readable to people outside the field.
Why his name is everywhere in the ancient-astronaut literature
Because of one book. From the Tablets of Sumer (1956), later reissued as History Begins at Sumer, is organised as a list of firsts: the first schools, the first legal precedent, the first pharmacopoeia, the first library catalogue, the first love song. That catalogue is the source both Zecharia Sitchin and Heather Lynn draw on for the argument that Sumer arrives too complete to be explained by ordinary development, and both cite it accurately.
What Kramer himself concluded
Nothing of the kind. The firsts are a popular framing for a corpus he spent fifty years editing, and his own explanation of Sumerian precocity is the ordinary one: a long Ubaid runway, irrigation agriculture, surplus, cities and the administrative pressure that produced writing. Using his catalogue while discarding his conclusion is legitimate, and a reader should know it is happening. He is also the translator behind a good deal of the Atra-Hasis Epic and lament material the alternative books quote.
Connections
Zecharia Sitchin · Heather Lynn · The Anunnaki Chronicles · The Anunnaki Connection · Atra-Hasis Epic · Enuma Elish · Anunnaki · Michael S. Heiser · Enki