The book I am looking for is actually a short story, but I am certain it was published in at least one collection of Isaac Asimov stories.
In the story almost all people acquired the ability to read by some fast and easy process, maybe by taking a pill or something. But there was a small proportion of children where that process did not work and they had to learn to read the old fashioned way. These children were considered unintelligent by their peers, most adults, and even themselves.