Q: So, if I live in Nigeria, I can't buy a book on iBooks and read it on my computer?

You can buy whatever is sold on the itunes store in your country.

You cannot buy from other countries itunes stores.

Ibooks are only for ipod touch/ipad/iphone.  You cannot read them on your computer in any country.

So, if I live in Nigeria, I can't buy a book on iBooks and read it on my computer?

It doesn't matter where you live.  iBooks cannot be read on a computer.  They can only be read on iOS devices such as an iPad.

Ashley,

Each country's iTunes Store has a different selection.  Apple has not explained why this is the case.

According to the official list ( http://support.apple.com/kb/ht4098 ) the Nigeria iTunes Store offers free books, but not books for sale.