In 1967, the author of ''One Flew Over the Cuckoo'''s Nest began serving a six-month sentence for marijuana possession. How ironic that the merry prankster who helped usher in the LSD-fueled Summer of Love spent that summer behind bars. But Kesey was anything but idle in lockdown. And now his prison diaries, profiles of fellow inmates, and psychedelic cut-and-paste collages are finally on view. During his hitch, Kesey sounds as quixotic as his ''Cuckoo'''s Nest alter ego R.P. McMurphy, writing to his wife: ''It's a drag in here but I'll survive, maybe even prosper.'' Prosper's an understatement. The rap on Kesey, who died in 2001, was that he never published enough to be considered a Great American Writer. This gorgeous, haunting book should help send his case to the appellate court.