Google led me to "Madre del cielo"... which kind of makes sense because "ama" means "mother", and "goia" means "top".
Amagoia is the mother of
Amaia in Basque mythology.
Amaia in that sense is like the Basque version of
Eve (the female counterpart of
Aitor, which is the mythological father of the Basques). I'm not sure Amagoia has any more meaning than being the mother of
Amaia.
I don't have any legit sources for any of this apart from Euskaltzaindia, the wikipedia entry about the book, basic knowledge of Basque and even more basic familiarity with Basque mythology, so I'm by no means an expert.