Fifty years after its publication, Ray Bradbury’s “Fahrenheit 451” remains “a favorite of teachers who assign it to English classes and librarians who appreciate its celebration of literacy as the hallmark of civilization. The public loves it, too. Last year, ‘Fahrenheit 451’ reached No. 1 on the Los Angeles Times best-seller list after Mayor Jim Hahn made it the centerpiece of a citywide reading program. Mr. Bradbury insists that the purpose of “Fahrenheit 451″ was not to prophesy. ‘I wasn’t trying to predict the future. I was trying to prevent it.’ In one immediate sense, he failed.”