You can take decorations off your Archive Bookshelf and return them to the Bookshelf Inventory, but NC decoration items cannot be removed from the Archive Bookshelf Inventory itself as the items are not tradeable. NC Bookshelf decorations go directly into the Bookshelf Inventory when they are purchased and cannot be removed or deleted.
This may change in the future if NP Bookshelf decorations are added, but for now items cannot be removed from the Bookshelf Inventory as they are only NC.