Actually, it's half of one of those '2 COMPLETE SCIENCE FICTION NOVEL' books you occasionally see in book stores; the other half of the book consists of Lin Carter's The Purloined Planet, which I will probably read soon but am unlikely to post a review of unless it's particularly extraordinary. Each of the 'complete science fiction novels' is about 100 pages long..
I reproduce the blurb on the back of the novel. As usual, it is inaccurate and completely misleading. Good old '60s science fiction copy!
Someone -- something had infected the minds of a group of unrelated people with an identical terror. A girl unnaturally kept from the world by a psychotic mother. . . . A man imprisoned for many years. . . . A socialite who enjoyed life's pleasures. Something had invaded their minds. Something evil. . . .
The main character's a guy who is trying to decide whether he should continue his current job as an editor or if he should go to medical school and become a psychologist, which is his current avocation. While at a party, he's convinced by the hostess to demonstrate some hypnotism tricks, and while he's doing so he accidentally induces hypnotism in one of the other guests who starts having disturbing hallucinations about being in a fantasy universe with dragons and whatnot. He determines that she's been hypnotizing herself and having these hallucinations for some time, and when he mentions this to his mentor in the psychological field he learns that a violent inmate at a local jail has been having identical self-hypnotized hallucinations. What connection can there be between them?
What with all the stuff about science fiction on the cover and in the blurb you might think that there's another dimension that they're channelling or something, but it turns out that this book is more of a thriller with psychological components a la Psycho, and (in my judgment) a pretty good one. The mystery is baffling, although enough hints are telegraphed along the way that when the explanation is presented it makes sense. (People who actually know some psychology would probably differ with me both in the plausibility of the explanation and about the book being non-science-fiction.)
So all in all, if this isn't one of Brunner's most memorable or innovative books, it's still a successful venture into a new genre and worth a read.