Legendary traits are started when you pick up a legendary book drop and you are level 39 or above. The book is removed from your inventory and you are directed to an NPC in Rivendell, Legolas for hunter legendary books.
Once you speak with the appropriate NPC, the book becomes a deed, and the pages, once put in your inventory will be removed and the deed is updated. Get all the pages and you can slot your trait after level 41. One Legendary slot opens at 41, the other opens at 45.
Books are random drops from sentient 2-legged creatures level 39 and above.
Pages drop from levels 35 and above, sometimes 4 pages from a particular book will drop from the same type (and only this type). Pages can drop from non-sentient mobs.
For his particular book, the pages may be random accross middle earth.