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To buy and download books, you need to have access to internet connection. So if you don't have WiFi there, then you should buy one that have 3G. If you only use it for reading books, you can get Kindle Paperwhite 3G. See here --> http://www.amazon.com/gp/product/B00QJDVBFU/ref=fs_cl
It also have WiFi. If you want a full featured tablet, then you can get Kindle Fire HD 4G/LTE. See here --> http://www.amazon.com/Kindle-Fire-4G-LTE/dp/B008GFRDL0
If you have any questions or would like any additional information, please don't hesitate to ask.