I love this book. I'd say that it's tied with Watership Down for the best book I've read so far this year.
You may be familiar with the story from the movie: R.P. McMurphy, a gregarious and "cagey" rascal, gets himself transferred to a sanitarium to escape a relatively short prison sentence. At the sanitarium, his outgoing personality clashes with that of Miss Ratched, the sadistic and fascist head nurse who runs the place with an iron fist. In the movie, McMurphy is the main character; the book is narrated by Chief Bromden, a massive Native American who pretends to be deaf and dumb, hanging out with the "Chronic," unfixable patients instead of the "Acute" fixable ones. Reportedly Kesey refused to see the movie because it wasn't narrated by Bromden, and one can see why: Bromden is pretty central to the book. He believes that a shadowy force called "The Combine" is slowly mechanizing all human beings, causing them to conform and think the same way. When the Combine can't mechanize someone, they go to the sanitarium, where more drastic measures are taken.
One of the central ideas of the book is that sanity is a false idea promoted by society to remove undesirable elements. This may seem silly, but the patients in this sanitarium, all based on people Kesey knew when he worked in a similar facility, include a "latent homosexual" and a stutterer whose only problem is that his mother has kept in an eternal childhood. The latter character, Billy Bibbitt, becomes the best example of Nurse Ratched's cruelty: as a friend of Billy's mother, she is able to control him utterly by insinuating that every time he tries to stand up for himself that she'll tell his mother about his insubordination.
The whole thing is told in spooky, surreal prose. The Chief repeatedly suggests that not everything in the book is real--and of course everything can't be, because there are some very odd depictions of life in the sanitarium that seem like they're taken from Dali paintings--but the presence of McMurphy, who is a racist and a sexist and a jerk but stands up for the patients against the tyranny of Nurse Ratched, brings a growing clarity that helps the Chief finally become a speaking member of the sanitarium's "society." This is one of those books that manages to be beautiful and horrible at the same time, and I love it.
Now I just have to see the movie.