You buy the book from vendors in the PoK library. YOu then turn the book in to your Guild Master. Hit alt-c to open the discipline window, and make a hot key. When you want to use the discipline, hit the hot key.
They are on a timer, so it will remain grayed out for a while, then you can use it again.
This space for rent, much like the space between my big, pointy elven ears... :)