His first books were the children's stories "The Silver Crown" (1968) and "Mrs. Frisby and the Rats of NIMH" (1971). He won the 1972 Newbery Medal for the latter and his comments at the American Library Association annual conference were read by his editor Jean Karl to preserve his anonymity. According to Sally Conly, his last two novels, "A Report from Group 17" (1972) and "Z for Zachariah" (1974), were intended for adults. After his death in 1973, Sally and their daughter Jane finished the latter novel using his notes so it could be published posthumously. "Z for Zachariah" received a 1976 Edgar Award for Best Juvenile Mystery from the Mystery Writers of America.