Reader for the Dead Tongue
Return to your capital city and seek out a warlock trainer who may know more about the codex.
Bring the codex to a Warlock trainer in your capital city.
Alliance warlocks go to the Slaughtered Lamb Inn in the Mage Quarter (Stormwind). Horde warlocks go to the Cleft of Shadows. Seems to be a random Warlock trainer. Visit trainers in other cities if you can’t find any in Stormwind or Orgrimmar with a yellow question mark above his head.
Sandahl: What … where in the great dark did you find this?