I read this when I was a kid, and it had a strong effect on me, at least in part because I had a next-door neighbor who was my age and mentally retarded. Now that I'm a parent it strikes me that we all follow the path of Charlie Gordon -- starting from modest intellectual ability, developing to a peak, followed by inevitable decline. It happens more gradually for us than for him, but we are equally aware of the process and where it leads.
Wikipedia: ... The titular Algernon is a laboratory mouse who has undergone surgery to increase his intelligence by artificial means. The story is told as a series of progress reports written by Charlie, the first human test subject for the surgery.
... Another key moment came in 1957, while [author] Keyes was teaching English to students with special needs; one student asked him if it would be possible to be put into a regular class if he worked hard and became smart.
... The procedure is a success and, three months later, Charlie's IQ has reached 185. However, as his intelligence, education and understanding of the world around him increases, his relationships with people deteriorate.
... Charlie discovers a flaw in the theories that led Nemur and Strauss to develop their intelligence-enhancing procedure. Shortly thereafter, Algernon starts behaving erratically, loses his new intelligence, and dies. As Charlie does further research, he determines that he too will inevitably revert to his old condition.
From the Wikipedia entry I learned the novel is based on a short story that appeared years earlier and won the science fiction Hugo award. The full text is here (at least for the moment). I think the novel is stronger than the short story.
Here's an excerpt (Google Books).
June 29 -- ... I phoned Landsdoff at the New Institute for Advanced Study, about the possibility of using the pair-production nuclear photoeffect for exploratory work in biophysics. At first he thought I was a crackpot, but when I pointed out to him the flaws in his article in the New Institute Journal he kept me on the phone nearly an hour. He wants me to come to the Institute to discuss my ideas with his group. ... That's the problem, of course. I don't know how much time I have. A month? A year? The rest of my life? That depends on what I find out about the psychophysical side-effects of the experiment. ...