YOU CAN FOOL ALL OF THE PEOPLE ALL THE TIME by Art Buchwald (Putnam's: $16.95). Art Buchwald is a potent force in American humor. By pointing his cigar-cum-magic wand at a person, institution or event, he can within the space of 700 words transform it into a laughing matter. In this, his 25th book, he has taken on the final installment of the Ronald Reagan presidency, concluding that it is even funnier than the first. Buchwald sets up his premises with a deft precision that can neither be blunted nor deterred by the rhetorical equivalent of the Star Wars defense: "In the world of nuclear arms," he says, "missiles do not kill people, nations kill people." "Now that the selling of the presidential inauguration has proved so successful," he asks, "can the selling of the presidency be far behind?" Buchwald pounces on politicians, self-interest, steroids, people who are professional Coca-Cola tasters and editors. His humor is mitigation of the Teflon presidency: He can go for the throat and get away with it. I belong to that percent of readers (my own poll) who would rather laugh at Buchwald than be mad at him.