Since I haven't read this one, I rely upon a review from Analog October 1970 for a decent plot summary.

"Clifford Simak seems to be embarked on a campaign to break down the barriers between SF and fantasy. In his last two novels, The Werewolf Principle and The Goblin Reservation, situations right out of the lamented Unknown were duly rationalized, and here we go again - this time with some variations of the concept argued by various philosphers, that the world is "really" what you - or someone else - think it is. Horton Smith, a writer seeking a quiet rural backwater where he can catch his breath, gets a note from a mysteriously dead friend: "I believe that man, with his imagination, with his love of story-telling, with his fear of time and space, of death and dark... has created another world of creatures which share the earth with him. Some day they may come out of their concealment." And so they do - and they are out to get anyone who knows about them, Horton Smith first of all. He is almost run down by a dinosaur. He spends the night with Snuffy Smith and Loweezie - Barney Google was away at the time. He is framed for murdering a nonexistent hillbilly lout. He is hunted by werewolves. He encounters Don Quixote. He gets some help from the Devil. And he is dogged across the country by a nasty little gnome called the Referee, who among other things gets him shot at the Battle of Gettysburg (The Gettysburg episode is the best in the book.) Unless you have strict ideas about what SF is, you may enjoy it." [-P. Schuyler Miller.]