The plot had to do with "astronauts" who travelled to other worlds telepathically rather than physically. On one such exploration, the protagonist encountered a big pink "thing," which somehow sort of mind-melded into his head. The astronaut/paraonaut spent much of the novel fighting this alien in his head. Near the end, he finally realized it might be more helpful than harmful.
I've been trying to identify this book for decades. Thanks for any help you can provide!