Humorist Josh Billings wrote in the 19th century, “There are two things in life for which we are never truly prepared: twins.” When Jane Roper found out she was pregnant with twins, she tried hard to find a memoir from another mother who could give her some advice. She didn’t find it, so she has written her own, “Double Time.” (St. Martin’s Press) It’s an up-close look at her first three years raising twin daughters. This hour, Charity speaks with Roper, who has an M.F.A. from the University of Iowa Writers’ Workshop and writes the popular “Baby Squared” blog on Babble.com.