The Kindle 3G works in a lot of Nigeria - go to this link, and then click on Global 3G to see the precise coverage:
http://www.amazon.com/Kindle-Wireles.../dp/B002FQJT3Q.
This would allow you to order books from Amazon (there are hundreds of thousands of free PD books at Amazon) from the device, without a computer. It *may* also allow you to use the web browser without wifi - that varies from country to country. Of course you can always browse if you have wifi, or just download books to your computer and sideload them with a USB cable.