Anderson & Riddersteadt. Investigation book: Who took this picture?

A book about a file I happen to have. File: «Stefan & Ragnhild Anderson coffee group_1961.jpg». This is one of the pictures of family dinners (two close families of Anderson and Riddersteadt). There’s an empty chair in the center of the frame, most likely the place of the man who took the picture. It’s an investigation book about a photographer who’s been unknown to anyone until now.