To command new books is done in three steps:
mark which cards are to be commanded. See the orange "shopping cart" button in many places of the software (card page, history of a day, search results…)
see the whole list, fine tune the quantities.
send the list to suppliers.
When we sell a card, if its quantity in stock goes below its desired treshld (which is 1 by default but can be set independantly), the card is automatically put in the list of cards to command.