Purchasing ebooks through the iBooks Store is very similar to buying any other media via Apple's iTunes Store. One key difference, though, is how you access the store. Rather than through iTunes or a dedicated app like the iTunes Store app on the iPad and iPhone, in this case, you access it through the iBooks app.
In the top left corner of the iBooks app is a button that reads "Store". To access the iBooks Store and begin shopping for iPhone- and iPad-compatible ebooks, tap that button.