You'll find three tables below. The first is a list of books you've not yet bought. This is based on orders you've placed through the online shop using the account you're currently logged in with.
The website has no way of knowing if you've bought books elsewhere. However, you can mark a book as 'owned' by clicking the heart icon in its row. That way, you can easily see which books you already own, and which you still need to complete your collection.
You can also add any books you want to purchase to your cart.
Below this is a list of books you've bought with this account. This is the inverse list of your 'to buy' list, but does not take account of items you manually marked as owned.
The final list does show books you've marked as owned. From here, you can 'unmark' them by clicking the heart icon again.
On some browsers, you may need to refresh the page after adding/removing books.
This feature is still experimental and is being continually improved, so please report any issues you find to firstname.lastname@example.org.