Once a Kindle is purchased and registered to an Amazon account, you can start buying books, downloading sample chapters, and "buying" the free books that Amazon offers.Harvey said:
So, if your wife has indeed bought you a Kindle for Christmas, then it may be registered to her--or your--account (if you have a joint account). On the other hand, if she designated it as a gift when she bought it, then it will not be registered to any account. The gift recipient (you) would need to register it upon opening it. In that scenario, you wouldn't be able to buy any books in advance.
Leslie