For what it's worth, Lovecraft himself had a rather extensive library. (See "Lovecraft's Library," assembled by ST Joshi, for details.) I've seen several references to his Grandfather's library as well, but I don't know the details.
Lovecraft had read W. Scott-Eliot's "Atlantis and the Lost Lemuria," (See page 178 of Joshi's "More Annotated HP Lovecraft"

and "The Call of Cthulhu" probably best represents Lovecraft's vision of Atlantis. It is interesting that in "The Last Test" (a revision by Lovecraft & Adolphe de Castro) one of the characters claims that "there are things in Alhazred's Azif which weren't known in Atlantis!"
Lovecraft may have been aware of Sumer, but he seems to have been more interested in ancient Egypt. See "Nyarlathotep," "Beneath the Pyramids," "The Nameless City," etc.