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 Mystcraft age. Linking books are crafted simply by putting a lone book in a crafting grid.
You can modify the linking book in the Link Modifier by putting the book you created into it and clicking the intralinking and following buttons. It is suggested you rename the book to where it teleports you at this time as well. When you return the book to your inventory, it will have an enchantment glow. And when you use it, you will be teleported to wherever you created the original link book. You use it by clicking on the black square in book. The book will remain in your inventory, and it does not matter what age or place you are in when you use it. You can use this to teleport to various locations around your world instead of walking there.
Linking Books are often overlooked by eager players, but more experienced players know better. Without a Linking Book, a player can be trapped forever on an age. Common practice is to place a Linking Book on a Bookstand as soon as the player links to a new age.