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These german books can be purchased via the following link as e-book at Amazon.de. For this, you do not need e-Reader! Perhaps you have an iPad.
You go to the App Store and the App invite free "Kindle" down. If you do not have a user account at amazon.de, so you can create  easily one.
From now on you can buy there any E-books. They are immediately available in the Kindle app.