Inventory-cost disappear

If you aren’t maintaining stock, that is, not using the Inventory Items tab then do you need to have that tab activated, if not, then disable that tab as it’s not required.

Now go to Setting > Chart of Accounts and create your own Inventory - cost account which you could call “Books Purchased”. Now you can select that account when entering the Purchase Invoices.

You could read this topic which may help with your situation: