Flowers for Algernon; by Daniel Keyes.

The story takes us on a journey that begins in lowlands of Charlie's mentally simpleton life, where he is barely able to sweep the bakery he works at, and study at a priliminary level with difficulty, always spelling his name with an inverted R - to the pinnacles of brilliance he achieves with a drug newly being tested that he has volunteered to be experimented with, which is successful. For a while.

Algernon is the mouse they are experimenting on simultaneously, and he becomes a pet, a friend for Charlie.

And an alter ego, in some sense. They are both subject to the same experiments, and success of one might indicate that of the other. And the other way around.

A heartbreaking tale in spite of the inevitability of much that saddens one.