It drops from Zakum. The skillbook is untradeable, but you can either use Scissors of Karma (SoK) from the Cash Shop to make it tradeable from someone who has picked it up at Zakum, or you can ask your friends if they would take you on a run to loot the skill.
If you go on a run, make sure you don't loot anything else, like the helms. Those are untradeable too, and you can really annoy people if you loot out of turn.