1. Open the App Store on your iPod touch.
2. Tap on the Search tab in the bottom right corner of the screen.
3. Type "Harry Potter" in the search bar at the top of the screen.
4. Tap on the "Books" tab in the top left corner of the screen.
5. Tap on the Harry Potter book that you want to purchase.
6. Tap on the "Buy" button and enter your Apple ID password.
7. Once the book has been purchased, it will be downloaded to your iPod touch.
You can open e-books on your iPod touch by tapping on the "Books" app.