Watchers of the Dark by Lloyd Biggle Jr.
Sinister, invisible forces of a secret mental weapon known only as The Dark are threatening the entire Primores galaxy, several transmitting leaps away from Earth. By the time a bizarre Mr. Smith comes to detective Jan Darzek's New York office, whole planets have been lain waste. Darzek is offered a million dollars by Smith to accept a job that will almost certainly be fatal: identify the incredible power that is about to overwhelm the few remaining planets in the beleagered galaxy, so that these worlds might somehow halt the rampage. A superb science fiction novel by a master of the genre!