Curator Erica Hirshler’s story behind the people portrayed in one of John Singer Sargent’s most famous paintings is turning out to be one of the surprise art book sellers not connected to an exhibition this season. Released in October by MFA Publications, the publishing arm of Boston’s Museum of Fine Arts, Sargent’s Daughters: The Biography of a Painting was out of stock in Dec. Strong reviews in The New York Times Book Review and the Boston Globe, among others, as well as an author interview on NPR’s The World have helped give it momentum.