The books he finds are uniformly old, often treating Phaerimm as a curious relic of the past. No book was more recently written than 1358 DR.
No one knows any of these things, nor have ever spoken or written of them. Phaerimm are a complete mystery, scholars, at best understand the Netherese fought and lost a war against them and that their power was so immense it destroyed the mightiest human empire in history and left it a barren wasteland.
Its likely they can not be overcome most scholars claim, and most the books Tharey reads imply the Phaerimm are extinct and were destroyed in the calamity that took Netheril when Mystryl died, its presumed the magical Phaerimm were all destroyed in this event.
Some theorize Phaerimm are adult carniphages, a specialized race of illithid savants, or extra-planar parasites that feed on magic. No one theory is widely embraced by any.