The Nexus book machine
The Nexus is a small Age which is essentially a mass transit hub that runs on Linking Books. The only features are the rotating walls, which house hundreds of Linking Books, and a large KI terminal. Once an explorer places their KI in the terminal, a list of Ages to which they have access appears on a screen. The explorer can browse this list, and choose an Age they wish to Link to. Once this choice is made, the terminal fetches the Linking Book indicated on the screen, and then opens it in front of the explorer. The explorer can then link to the Age they have chosen, or choose a different Age. The KI remembers any D'ni Ages they have been to, their neighborhood, and any of the City terminals that have been activated with that KI.
The DRC had access to a separate area of the Nexus, which contained the control hardware and software for not just the Nexus itself, but also many of the KI's networking functions. Referred to as the Lattice, not much is known about it, other than the fact that it has served as a constant source of frustration for Dr. Victor Laxman, the primary DRC member responsible for the KI's operations.