Land of the blind - Jess Walter

Burned out and working the weekend night shift, Caroline Mabry is confronted with the arrival of an apparently unstable but charming derelict who announces he wants to confess to a murder.
At first she dismisses him as a nutcase.
But when she realizes he is none other than former wunderkind politician Clark Mason, she agrees to let him write out his confession.
Nineteen hours later, with Mason still feverishly writing, Caroline finds herself scrambling to investigate his long, progressively darker and more sinister tale.