How do you write the books are German in Spanish?

The most straightforward translation of "The books are German" in Spanish depends on what you mean by "German".

* If you mean the books are written in German: "Los libros son en alemán."

* If you mean the books are from Germany: "Los libros son alemanes." (This is more natural than "Los libros son de Alemania.")

So the correct translation depends on the intended meaning.