Book I bought is in German. Can I change it to English?

What language was the book listed as being in on its description page in the store ? If it's incorrectly labelled then you should be refunded, if it's correctly labelled then Support may refund you.

You can try the 'report a problem' link to contact iTunes Support (and explain that you wanted it in English) and see if they will refund or credit you so that you can buy the English version (assuming that it's available in your country's store) : http://reportaproblem.apple.com

If the 'report a problem' link doesn't work then you can try contacting iTunes Support via this page : http://www.apple.com/support/itunes/contact/ - click on Contact iTunes Store Support on the right-hand side of the page, then Purchases, Billing & Redemption

You may need to look for the book in a different store, like Kindle, Nook, Kobo, Google, or Sony. They all have ereader apps for the iPad.