Kumabjorn, you're right that Amazon will apply your gift voucher automatically when you make a 1-click purchase.
But that only applies to Amazon in the US. With Amazon UK gift vouchers, the small print specifically disallows them for 1-click purchases (I'm not sure about other countries.)
Re your other question: No, I don't think you can buy a book as a gift (although I don't know why not). You would have to give a gift voucher as a gift, which isn't the same thing at all.