ATTACK OF THE SQUIDY HAIRBALL
The cover copy reads as thus, according to all-helpful Wikipedia:
On the face of the Earth only the Barrenland remained an impenetrable mystery—a blasted radioactive area the size of a small state where no man dared to venture. But Jervis Yanderman was one of the more courageous souls of that future day when men at least were starting to reconstruct the vanquished civilization of the dim past.
Jervis knew that the secret of the Barrenland had to be solved. For things from out of this world still emerged from it to terrorize neighboring lands and strange weird visitations haunted those who even approached it.
So they ventured inside to find the people who had creatued the civilization that resulted in the blasted barren area with its horrible creatures. It had ESP and an inbred elite and lusty questing fellows who spoke in Thees and Thous. This would be tiresome eventually, but if you got in with those ideas before everyone else, you had something.
I think Poul Anderson got in there.