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A Linking Book creates a link to the spot in which it was created, providing players a way back home should they decide to link to a new age, or convenient shortcuts to specific locations in another dimension (or even the same dimension, at additional cost).
Linking books are created by combining leather and a Linking Panel, and start off unlinked until you right click it, at which point it links to the spot you are standing at that moment. Prior to 1.5, linking books were crafted simply by putting a lone book in a crafting grid, and linked to the spot you were when you crafted them.
You use it by right clicking on the book to open its interface, and then clicking on the black square in book. Again, this will only work if the linking book links to another dimension, unless the book has Intra Linking. If you were holding the book in your inventory, the book will fall to the ground where you were standing, where it may then be subjected to environmental damage or destruction. To avoid this, the book can be safely stored and accessed from a Bookstand or Lectern. An additional book modifier, Following, exists to keep the book in your inventory rather than dropping it, but this modifier is currently unable to be added to books without cheats.
Linking Books are an essential, yet easily overlooked, tool when writing ages, as without a Linking Book back home, the player can be trapped forever in an age.
The book will by default have the name of the age that it was created in, which you can see in the item tooltip text or in the interface, just below the book icon. You can change this name in a Writing Desk. Notice that there will then be no other indication of the age that the book links to, aside from actually using the book and observing the age.
1 Leather + 1 Linking Panel = 1 Unlinked Linking Book
Right click with the Unlinked Linking Book to link it to your current spot and create a Linked Linking Book.