The side quest "Unspeakable Secrets" will continue to update. Just keep following the main storyline for now.
There's nowhere else to go nearby, so let's continue with the main quest. From where we just fought the boss, head forward; there's a path on the left that leads upward.
After coming up here, there's Dust behind this chest.
Continue forward and you'll see Zhui Ming again. After he leaves, explore this area and pick up nearby items first.
You can obtain the book ["Four Fiends Axe" Case File #6: Captain Wu's Last Letter] from the table.
