ON THIS WIKI
The Link Modifier has always been and will always be a debug block, not meant for gameplay. The things it offers will be available through other means. 
As of Mystcraft version 0.10.0, the recipe for the Link Modifier no longer functions, but it can still be spawned in Creative mode. The author states that book linking features (i.e. Intra-linking, Following, etc.) will be added in future updates.
Intra-age linking: Allows using a linking book that was created in the same age you are in. Useful for creating teleports between different areas in an age.
Following: Keeps the used linking book in your inventory, opposed to dropping it when used. Crucial for using linking books to teleport around.
Generate Platform: Creates a small platform under the player once teleported. Useful for void worlds.
Retain Momentum: Allow an entity to retain their movement speed when teleported. Mainly used for crystal portals.
Disarm: Drops everything in the player's inventory when the linking book is used.
From Mystcraft 0.10.0 on, the Link Modifier can no longer be crafted, but the block still exists and functions, so it can be creatively spawned in.