Flowers for Algernon by Daniel Keyes

Until he was thirty-two, Charlie Gordon --gentle, amiable, oddly engaging-- had lived in a kind of mental twilight. He knew knowledge was important and had learned to read and write after a fashion, but he also knew he wasn't nearly as bright as most of the people around him. There was even a white mouse named Algernon who outpaced Charlie in some ways. But a remarkable operation had been performed on Algernon, and now he was a genius among mice. Suppose Charlie underwent a similar operation...