I don't know why he was so fond of it. The Man Who Japed was originally half of an Ace Double, so it could pass as a novella. It is also just one of about five book-length works Dick wrote or put under copyright in 1956. Familiar PKD elements are all in place: a postwar dystopian future, a lone hero going against the code, incredibly fast pacing, digs at psychiatry, a brief trip to another planet. This is a moral world where the morality is enforced by neighborhood watch societies headed by middle-aged women in floral print dresses. (Such beings seem to be a particular horror to PKD. They show up in Eye in the Sky as well,) The ladies get their information from "the juveniles," two-foot-long mechanical centipedes charged with keeping a watch on things. Alan Purcell is part of this system. He works in a form of advertising that broadcasts campaigns with moral lessons that are good for the populace. During the course of the book he falls very afoul of the system and plots to overthrow it.
Satire and action here are good, but Dick's most prescient insights here have to do with real estate. In the 22nd century people with tiny apartments close to the center of town live in fear of code violations that will exile them to the what I suppose are tenements or something unpleasant further out.