Latest iBooks 3.1 for iPhone, iPod Touch and iPad is released with improvements as now it includes Japan with hundreds of thousands of books available for purchase, including fiction, manga, light novels etc, and also includes a number of improvements for reading Asian language books.
You can download iBooks 3.1 for iOS via direct links to iTunes Store below.
Download iBooks 3.1 for iPhone, and iPod Touch
Download iBooks 3.1 for iPad