Enter Amaro's hidden library.
After climbing up the grappling hook and over the balcony, you will be in an area with a bunch of plants. Do the following:
Open the door to the study (left of the tree with sap on it):
- Panel piece - In the plant area, spin around until you see a fallen tree. Pick this up from underneath it.
- Spin around until you find the door missing the handle. Use the brass handle on the door to open the study.
- Use the panel piece on the puzzle next to the door in the right side of the study room. Complete the puzzle.
- ACHIEVEMENT UNLOCKED when you enter the library room.