Sarah Jessica Parker is doing her homework. Named in January as creative director of the Halston Heritage label, the secondary Halston line featuring archival looks, the “Sex and the City” star was seen walking in the Village on Tuesday carrying a copy of “Simply Halston,” the 1991 best-selling biography of the late designer by Steven Gaines. As Richard Johnson of this page blurbed on the cover, “Stars, sex, drugs, this book has it all!”