Clifford D. Simak 
Out of Their Minds 

Out of their minds and the force of their imagination, men have created countless beings, from demons and monsters of legend to comic-strip characters. What if their world were real – if dragons, devils and Don Quixote hobnobbed with Dagwood Bumstead and Charlie Brown? Such a world would have its fascinations . . . and its dreadful perils – if it existed. Horton Smith found out that it did – and that he was right in the middle of it!