Mephis: There isn't really much directly referencing the pyramids at Giza in any written sources until the Hellenistic period and Herodotus' account. That's not really unusual for ancient buildings. The largest tomb known in Greece, and what appears to be a major cult site, is currently under excavation at Amphipolis. Nobody seems to have bothered mentioning it in any works about Macedonia or Thrace or Greece or even the city itself.
And, there are few clues inside the building to allow a conclusion as to the building's function. But, one of the more intriguing theories emerged recently.
Head archaeologist, Katerina Peristeri and her head architect Michalis Lefantzis recently said they had found three inscriptions with the word 'parelavon' (received) and Hephaestion's monogramme.
And, Hephaestion was Alexander the Great's boyfriend (lover) and had been so since they were young. Their tutor Aristotle described their friendship as "... one soul abiding in two bodies" (quoted from Diogenes Laertius 5.1.20 ).
It is known that when Hephaestion died, Alexander was deeply affected. There is evidence for that among ancient sources, for example, Plutarch says
"... Alexander's grief was uncontrollable ..."
and adds that he ordered many signs of mourning, notably that the manes and tails of all horses should be shorn, the demolition of the battlements of the neighbouring cities and the banning of flutes and every other kind of music. (Plutarch 72.3)
And Arrian relates an account that,
"... he flung himself on the body of his friend and lay there nearly all day long in tears, and refused to be parted from him until he was dragged away by force by his Companions ..."
"... for two whole days after Hephaestion's death Alexander tasted no food and paid no attention in any way to his bodily needs but lay on his bed now crying lamentably, now in the silence of grief."