A new book explores the rich heritage of five Delaware-area gardens that belonged to members of the du Pont family. “Du Pont Gardens of the Brandywine Valley” (Monacelli, $60 hardcover) looks at Hagley Museum and Library, Nemours Estate, Mt. Cuba Center, Winterthur Museum, Garden & Library, and Longwood Gardens. Written by Marta McDowell and featuring photos by Larry Lederman, the …