To get the quest, talk to Little Nine that is just north of Bishui Plains teleport waypoint in Wuwang Hill, near the stairs. You will be asked to find her book in the forest.
Find the book in the woods
The first one is not that far from her. Go further into the dark forest and near the stairs, you will find something on the ground that you can interact. If you can't find it, using elemental sight (middle mouse button on pc, or the eye icon on top left) can maybe help.
Go further in to the forest and you will find the other one in between Hidden Palace of Zhou Formula and the Bishui Plain teleport waypoint.
Completing the quest
Once you find both of them, go back to where you found Little Nine and interact with the object on the floor to complete the quest.