She will need an amazon.com account which she can set up ahead of time, but that might spoil the surprise. Not to worry, though...it should only take about 5 minutes to set it up on Christmas morning

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You can buy amazon.com gift cards at a lot of places (Target, Walgreen's and CVS come to mind) and use those to buy the books. Most of the books are priced anywhere between $1 and $10, depending on what you want. There is also a large selection of totally free books. Many local libraries also offer Kindle books for check-out, but you have to check your library's website for that information.
I had bought several Magic Treehouse books and put them on my Kindle so my DD8 could read them. With this deal, I bought her a Kindle of her own. I opened it up today and registered it to my account. By doing that, I was able to transfer all of the kid's books and games to her Kindle and I can remove them from mine.
I only wish that they had the Wi-Fi version (no 3G) since I really don't want DD to access the
amazon store without my knowledge, which she can do with the 3G.