After leaving the dungeons of Nchardak, you will be attacked by a large dragon - Krosulhah. Neloth will help you in the fight, so it shouldn't take last too long (or be particularly difficult). In return for defeating the beast, you will be able to absorb its soul and unlock the second Word of the Bend Will Shout - it will come in handy later on in the game.
Move to the Skaal Village - either with fast travel or on foot. After getting there, look for the shaman - Storn Crag-Strider. Tell him everything that you know, hand over the Black Book and watch what happens. Remember - from now on, until the end of the main story of the game, you won't be able to complete any mission for the Skaal (they won't speak to you)!