Timothy Mark Brownfield, 40, who allegedly mailed threatening letters to the FBI about Moon Unit Zappa, 32, daughter of late rocker Frank Zappa, pleaded innocent to federal charges in a Southern California courtroom yesterday and was transferred to a psychiatric jail facility. Brownfield had vowed to kill Zappa, said the FBI, because he claimed that she and her father stole lyrics to their 1982 hit song "Valley Girl" from him. Brownfield faces up to five years in federal prison if convicted (his charge is sending a threatening communication), according to the U.S. Attorney's Office. Frank Zappa died of prostate cancer in 1993. He was 52.