I'm guessing maybe Marquis de Sade's book that inspired the film, 120 journées de Sodome, would be as disturbing... maybe more...Guys please, it can't get more disturbing than Pasolini's Salò o le 120 giornate di sodoma! It has absolutely everything in it, methods of torture that I can't even explain.
JT Leroy's books are pretty disturbing as well.