Limited Collectors' Edition C-48 (Oct/Nov 76) was another issue I really loved. It reprints the two Superman/Flash races that were released in 1967, the first from Superman, the second from the Flash. The stories contrast surprisingly well.
Between the stories there is a four page spread of Superman's Fortress of Solitude, with labelled maps and views of the different parts.
The Bottle City of Kandor is given special focus in the part dedicated to the second level.