"Flowers for Algernon" by Daniel Keyes

This book is called science (or speculative) fiction, but that's far from the central theme which is about human relations, compassion, meaning of life and the justness of the world.

A developmentally challenged individual, Charlie Gordon, 32, has an IQ of 68 and works as a janitor in a bakery, but has an enormous wish to learn and become 'smart.'  He is recommended by his teacher at Beekman College school for retarded individuals, Alice McKinney, for an experimental surgical enhancement of his intelligence.  The researchers at Beekman College already performed the surgery and enzyme therapy on a mouse named Algernon, and he outperformed any mouse in the lab.

After the experiment, eventually, Charlie gets a genius IQ of 185 and even starts doing research on the topic himself (and writes a piano concerto), but is abandoned by his old friends and circles, as they only used him to make fun of him and feel themselves smarter in comparison.  His memories from childhood traumas come back and he has problems emotionally integrating them, as well as starting romantic relationships.

Eventually he discovers that his condition will revert and he will go back to IQ 68 or even lower - and writes a scientific paper to prove this.  His attempts to start a relationship with Alice eventually works, but when he sees his IQ dropping precipitously, he sends her away.  Eventually he reverts ack to his old IQ, but now finds that he cannot resume his previous life and job, so he commits himself to the Warden Institute for retarded individuals. The mouse Algernon dies, in a harbinger of the reversal of the condition that later happens to Charlie, and Charlies last words are not to forget to put fresh flowers on Algernon's grave.

A touching and powerful book about choices, ethics and the human condition.  Several publishers tried to force a happy ending on the author, but the sad ending makes the story much more powerful.