Stephen King - Thinner is a novel first published in 1984. Thinner by Stephen King was originally released under the pen-name of Richard Bachman, like a number of other Stephen King books.
Thinner - Stephen King book description:
Billy Halleck commits vehicular homicide when his lack of attention to driving results in the death of an old lady on the street.
Overweight Halleck is a lawyer with connections, though, and gets off with a slap on the wrist. After his trial, a gypsy curses him with a single word, "Thinner." Halleck begins to lose weight uncontrollably and must pursue the band of gypsies who are responsible for his dwindling condition.