Pieces of me! Ashlee Simpson may soon join her sister, Jessica, on the best-seller list. The “Little Miss Obsessive” singer, 35, is gearing up to spill everything, from the truth about her failed marriage to Fall Out Boy’s Pete Wentz to details about her infamous Saturday Night Live performance, in an explosive new memoir. “Jessica isn’t the only one with secrets to tell,” insists an insider. “Ashlee’s plastic surgery, her parents’ divorce, her exes — it will all be addressed.” The pop star, who once dated Wilmer Valderrama and Ryan Cabrera, shares son Bronx, 11, with punk rocker Pete and daughter Jagger, 4, with husband Evan Ross. “Of course, she’s also going to write about her mother-in-law, Diana Ross,” teases the insider, although a rep denies our story, “and the…