Why you are stuck at 2399/2400
Most stuck cases are caused by two books being placed in the wrong numerical order on a shelf (for example, Volume 4 placed before Volume 3). Because the books are shelved, spells like Assemble and Insight ignore them.
Use unlit section plates
Completed shelves glow bright blue. Identifying which section plate is unlit points you at the most likely error. Floor 1 sections have a 25% higher error penalty due to visual clutter.
Staircase Vase Recall
If you have fewer than 10 books left and the staircase vase is active, click the vase to teleport the remaining loose books to your position. This bypasses the manual search for misplaced books.
Database Sync Error
If you have shelved 2,399 books but all 16 section plates are glowing blue, the database has hit a synchronization error. Take all books off the nearest shelf and replace them to force a server update.