The Shrinking Man.

While on a boating holiday, Scott Carey is exposed to a cloud of radioactive spray. A few weeks later, following a series of medical examinations, he can no longer deny the extraordinary truth. Not only is he losing weight, he is also shorter than he was. Scott Carey has begun to shrink. Richard Matheson's novel follows through its premise with remorseless logic, with Carey first attempting to continue some kind of normal life and later having left human contact behind, having to survive in a world where insects and spiders are giant adversaries. And even that is only a stage on his journey into the unknown.