You may not find a lot of books available on Apple iBooks store, but there are some free books available, like Winnie the Pooh and Wuthering Heights.
The iBooks application already available at App store, but it doesn't work with iPhone OS 3.x or earlier. There might be someone who told you that you have to get an iPhone 4 to have iBooks, but it is not true. You can upgrade the OS to iOS 4 if you are using iPhone 3G or 3GS, I already tried that, is easy.
For sure if you get an iPhone 4, it already came with iOS 4, and you can just download iBooks. It just sounds much easier for some people.
So, enjoy your iBooks on iOS 4.