The inventory of the bookshop is done is 3 steps:
Be sure to have a barcode scanner that scans very quickly. This makes a difference.
Note: the Inventory menu was dramatically changed in Abelujo in March, 2023. It is now simpler and much faster, with the caveat that you can't easily see the difference between the stock of before and the real one. The URL to your old inventories is still accessible.