Are all eBooks in the iBookstore DRM protected? A few weeks ago I bought a book in the iBookstore. The title of this book is "Der Aurora Effekt" (German). Then a few weeks later I bought the book "The Tributes of Panem" (German version). 

The first title I can copy to my Kobo eBook reader and read the book. But "The Tributes of Panem" was not readable!

Is it possible that not all books are DRM protected?
Yes.

(Not all books are DRM protected.)

Per the official iBooks FAQ...


  How can I determine which books are copy-protected (DRM) versus those that are DRM-free?
  
  iTunes can show you the kind of book you have by selecting the book and then choosing File > Get Info.
  
  
  If the Kind is marked as Book, this is a book you downloaded from the Internet.
  If it is listed as Purchased Book, it is a DRM-free book downloaded from the iBookstore. 
  If it is listed as Protected Book, it is a copy-protected book from the iBookstore.
  


Only "protected books" have DRM protection.