These are the issues in which Mort Weisinger took over as editor and began introducing more components to the Superman “family”. The Earth-1 Fortress of Solitude was introduced in Action 241 followed in short order by Supergirl, the bottled city of Kandor, Brainiac, etc.
The purpose of the Superman “The Man of Tomorrow” series of hardcover Archives books was to showcase the earliest Silver Age issues and the first volume begins with these issues, as did the black and white Showcase Presents Superman softcover book.