what was the motivation and the purpose of the book burnings in Germany.

Answer 1

The purpose of the book burnings was to prevent Germans what the Nazis thought was subversive.

http://www.google.com/#q=+purpose+of+the+book+burnings+in+Germany

Answer 2

why where they called
"un-German" books,

Answer 3

The Nazis believed that they were the only "true" Germans. Anything they disagreed with was un-German.