The “Agents” of the title are presumably the espionage operatives for different agencies who provide the primary viewpoints in this short novel: the Signalman fatigued by his encounters with Things Men Was Not Meant to Know, and Immacolata Sexton exalted as a time-loose spectator. The latter seemed a little awkward to me; she was not really mysterious enough considering how unexplained were her superhuman abilities. I wonder if she features in other stories by Kiernan.
Still, the real meat of the book is in the business being investigated: Lovecraft’s “The Whisperer in Darkness” meets Charlie Manson and the Heaven’s Gate suicide cult in a southern California nightmare. The tale pulls that off very well, and I wonder–are these cultists really the agents? In that case “Dreamland” is Yuggoth. It’s certainly not the Dreamland of Lord Dunsany and HPL.