I see now that I was kind of mistaken. It was in my Kindle Wish List where I've put some books I am interested in. In order not to go nuts with 1-Click buying I've bought myself some Amazon Gift Vouchers, hence purchases will be deducted from a positive balance.
However, this got me thinking. Is it possible for me to buy a book for Kindle and have it delivered to someone else as a birthday gift? Pre-set delivery time, kind of thing.