Flowers for Algernon by Daniel Keyes
Charlie Gordon works in a bakery doing the most menial tasks. Nowadays we'd call him mentally challenged (or possibly a more specific diagnosis) but in the book (published in 1966) he is described as retarded. Even though he lacks many skills, he has the ambition to be more. He goes to special classes to help him read and write better. The teacher recommends him for experimental surgery and therapy that may increase his intelligence. The scientists have succeeded in making a mouse named Algernon smarter. The mouse can solve mazes with skill and speed. When Charlie is first brought to the lab, the scientists give him a paper-and-pencil version of the same maze that Algernon is about to run. The mouse beats Charlie. Charlie doesn't give up--he agrees to the experiment. His intelligence is increased, even beyond the level of the experimenters. But then Algernon shows signs of deterioration. Will the same thing happen to Charlie? Can he avoid losing his new-found intelligence?
The story is told through Charlie's journals. He begins writing them at the request of the scientists. The first entries have the poor spelling and grammar characteristic of Charlie's state. They also reveal a little bit of his family history and how he's been treated by his co-workers at the bakery (not too well). The transformation into an intelligent person is believable as is his character development as he learns more about the world and himself. Smartly, the story recognizes that his emotional development is not considered in the experiment, leading to problems for Charlie in dealing with others. He realizes intellectually how few people have treated him like a person and becomes a bit difficult and taciturn, in part because his emotional development doesn't match his intellectual development. He tries to reach out to his estranged family but has a difficult time there as well. His eventual slide back into a less intelligent state is heart-breaking.
The story is very compelling and well worth reading. It's also the topic of discussion at A Good Story is Hard to Find podcast #154.