From an author's perspective, this is one of the finest books with an unreliable narrator I've ever come across.
The story, set in a mental hospital, is told by Chief Bromden, or Chief Broom as he's called by the staff. Chief Broom is a huge Indian who never says a word, so he's assumed to be deafmute. He's not, but he is mentally disturbed, which colour his poetical descriptions of what happens in the mental hospital wing under the leadership of the strict Nurse Ratched.
When Randall McMurphy enters the mental hospital to avoid going to prison, his internment in the wing under Ratched leads to a direct confrontation. McMurphy aims to overthrow the strict rule and give the inmates back their life and dignity.
Chief Broom identifies the rebellious McMurphy with his own father, a huge strong Indian made weak and powerless by the oppression of white people. They become friends and Bromden gives an intimate account of how McMurphy tries to overcome the oppressive world Ratched has created, victimizing her patients, but the outcome is tragic.
Kesey's ability to enter into the mind of a disturbed individual like Bromden is impressive and, while I enjoy the film immensely, I regard the book as one of the few masterpieces of storytelling.