Detective John DiNatale signed his book, “Family Business,” at West on Centre in West Roxbury Tuesday night. DiNatale’s dad was a Boston detective during the terror in the 1960s sparked by the serial killer nicknamed the Boston Strangler. John kept the letters that confessed killer Albert DeSalvo wrote to his father. Last summer, DNA evidence from his last crime scene proved conclusively that DeSalvo was, indeed the Boston Strangler. The younger DiNatale and his brother Richard own a Boston detective agency.