Serialized as "The Fisherman" in Analog, April through July 1961. This is one of my favorites by Simak - one of his 5 best books. For a decent plot summary, I rely upon a review from Magazine of F&SF February 1962.

"...Simak has invented an interesting situation. Because of radiation barriers, man finds it is impossible to get off Earth and survive. Physically, he is barred from the stars. But he has devised Operation Fishhook, a technique whereby explorer's minds can be thrown thousands of light years out into space to explore the stars and their planets. The danger of this, as Fishhook explorer Shepard Blaine discovers in the very first pages, is that there are alien creatures in space who can sense the presence of these explorers from Earth, and parasitize their minds, giving them paranormal powers. (Hi, pal, it said. I trade with you my mind.) And on an Earth which already has a telepathic civilization, such paranormal men are feared and hated. The novel then develops into a battle between the benignly contaminated Blaine, and the operatives of the reactionary Fishhook, with all Earth thrown into an hysterical frenzy. The story is, of course, in the conventional persecution pattern; but we must point out again, with pleasure, that Mr. Simak is the one SF author who consistently refuses to adopt the cliché that aliens are, ipso facto, enemies. It reveals a sane and admirable aspect of his character; he is genuinely civilized." [-Alfred Bester.]