Each job has 3 legendary traits they can earn. Each one requires a book to start. Books are dropped off any sentient mob level 39 or higher (things like hill men, orcs, Iron crowns, etc). They are random, insanely low drops so you'll test your patience if you try to get them without an expensive trip to the AH.
The break down works like this:
One book has pages that are tradable and can be put in the AH. these can drop to anyone at anytime from the same mobs the books drop from.
The other two books have pages that require a quest to be opened. When you get one of these 2 books you have to travel and talk to a specific NPC (legolas in the case of hunters). That NPC will then tell you where to look for the pages. As a rule each of these books will require 8 pages and you will find 4 pages in one area and 4 in another. These pages ONLY show up once you have opened the quest at the NPC. Much like other quest items they are only available to those with the quest open. So you taking friends wont cost you lost pages. So to finish the last two books requires a total of 16 pages from 4 different areas/mobs.
Side note, you get 3 traits, but only 2 slots. Decisions, decisions.