The Nexus is the central hub of all the locations you have visited in the game and the means of visiting places with other explorers, plus your own Ages. The book is added to your shelf when you use it for the first time in various locations throughout the Ages. Each explorer has their own Nexus, and the book is one of the few that cannot be shared on their Relto shelf. You will need your KI from Gahreesen before you can access the machine.
The Nexus Menu
When you insert your KI into the device, it brings up a list of locations in the cavern and of the Ages you either have yourself or have been invited to.
When someone sends you an invite over the KI, the way to get to their Age is through this list. Simply search for the explorer's name and the name of the Age they sent the invite for, and you're off! (example: Explorer's Teledahn)
A list of various neighborhoods in the cavern. Note that some are called "hoods" while others are called "Bevins". No matter which they have, they will link you to similar-looking hoods, or "Bevin-style" hoods. Clicking on the "Name" or "Population" headings at the top will sort the list. Keep clicking for either ascending or descending sorting. Not all hoods are listed.
Links to the personal instances of your own Ages. A supplement to the Relto Shelf. Note that the links here only go to the default link-in spot of each Age.
Listed at the top is the name of the Neighborhood you've joined, created, or started with. Next to that is a button that will make your hood private (off the Nexus' Public Links list) or public (on the Public Links list).
To bump your hood to the TOP of the Nexus Public Links list, simply make your hood private, then public again. Do this each time you would like your hood at the top of the list.
If you would like to create your OWN hood, you will need to link to your Relto and press the button at the bottom of the hood book (the square with a dot in it, D'ni numeral for 0). The hood book will sink back into the shelf. Next, link back to your Nexus. You will notice a book icon where your hood was once listed. Click it to create a hood. It will be named after your avatar, so if your avatars name is Tom, it will create "Tom's Hood". Remember you can name your avatar almost anything you wish (within reason), so you can use one of your avatar slots to create an avatar to name a hood something specific.
For information on how to join people to your new hood, or to join a new one yourself, see Neighborhood.