Unspooling long-held family secrets has become a specialty of Dani Shapiro’s. She has hosted the popular “Family Secrets” podcast for several years, and her five published memoirs have often explored suppressed knowledge within her own family. With “Signal Fires,” Shapiro returns to fiction for the first time in 15 years, but she has not strayed far in terms of subject matter. Her new novel probes the long-term cost of secrets, as viewed through the Wilfs, a nonobservant Jewish family in a New York suburb.